hibernate - relational persistence for idiomatic java · 21.5. schemavalidator command line options...

HIBERNATE - Relational Persistence for IdiomaticJava

Hibernate Reference Documentation

The Hibernate Team

The JBoss Visual Design Team

4.3.4.Final

Copyright © 2004 Red Hat, Inc.

Legal Notice

2014-03-03

Table of Contents

Preface

1. Tutorial

1.1. Part 1 - The first Hibernate Application

1.1.1. Setup1.1.2. The first class1.1.3. The mapping file1.1.4. Hibernate configuration1.1.5. Building with Maven1.1.6. Startup and helpers1.1.7. Loading and storing objects

1.2. Part 2 - Mapping associations

1.2.1. Mapping the Person class1.2.2. A unidirectional Set-based association1.2.3. Working the association1.2.4. Collection of values1.2.5. Bi-directional associations1.2.6. Working bi-directional links

1.3. Part 3 - The EventManager web application

1.3.1. Writing the basic servlet1.3.2. Processing and rendering1.3.3. Deploying and testing

1.4. Summary

2. Architecture

2.1. Overview

2.1.1. Minimal architecture2.1.2. Comprehensive architecture2.1.3. Basic APIs

2.2. Contextual sessions

3. Configuration

3.1. Programmatic configuration3.2. Obtaining a SessionFactory3.3. JDBC connections3.4. Optional configuration properties

3.4.1. SQL Dialects3.4.2. Outer Join Fetching3.4.3. Binary Streams3.4.4. Second-level and query cache3.4.5. Query Language Substitution3.4.6. Hibernate statistics

3.5. Logging3.6. Implementing a NamingStrategy

3.7. Implementing a PersisterClassProvider3.8. XML configuration file3.9. Java EE Application Server integration

3.9.1. Transaction strategy configuration3.9.2. JNDI-bound SessionFactory

3.9.3. Current Session context management with JTA

4. Persistent Classes

http://www.hibernate.org

http://hibernate.org/Documentation/DocumentationOverview

http://hibernate.org

http://design.jboss.org/

ln-d5e19.html

legalnotice.html

4.1. A simple POJO example

4.1.1. Implement a no-argument constructor4.1.2. Provide an identifier property4.1.3. Prefer non-final classes (semi-optional)4.1.4. Declare accessors and mutators for persistent fields (optional)

4.2. Implementing inheritance4.3. Implementing equals() and hashCode()

4.4. Dynamic models4.5. Tuplizers4.6. EntityNameResolvers

5. Basic O/R Mapping

5.1. Mapping declaration

5.1.1. Entity5.1.2. Identifiers5.1.3. Optimistic locking properties (optional)5.1.4. Property5.1.5. Embedded objects (aka components)5.1.6. Inheritance strategy5.1.7. Mapping one to one and one to many associations5.1.8. Natural-id5.1.9. Any5.1.10. Properties5.1.11. Some hbm.xml specificities

5.2. Hibernate types

5.2.1. Entities and values5.2.2. Basic value types5.2.3. Custom value types

5.3. Mapping a class more than once5.4. SQL quoted identifiers5.5. Generated properties5.6. Column transformers: read and write expressions5.7. Auxiliary database objects

6. Types

6.1. Value types

6.1.1. Basic value types6.1.2. Composite types6.1.3. Collection types

6.2. Entity types6.3. Significance of type categories6.4. Custom types

6.4.1. Custom types using org.hibernate.type.Type

6.4.2. Custom types using org.hibernate.usertype.UserType

6.4.3. Custom types using org.hibernate.usertype.CompositeUserType

6.5. Type registry

7. Collection mapping

7.1. Persistent collections7.2. How to map collections

7.2.1. Collection foreign keys7.2.2. Indexed collections7.2.3. Collections of basic types and embeddable objects

7.3. Advanced collection mappings

7.3.1. Sorted collections7.3.2. Bidirectional associations7.3.3. Bidirectional associations with indexed collections7.3.4. Ternary associations7.3.5. Using an <idbag>

7.4. Collection examples

8. Association Mappings

8.1. Introduction8.2. Unidirectional associations

8.2.1. Many-to-one8.2.2. One-to-one8.2.3. One-to-many

8.3. Unidirectional associations with join tables

8.3.1. One-to-many8.3.2. Many-to-one8.3.3. One-to-one8.3.4. Many-to-many

8.4. Bidirectional associations

8.4.1. one-to-many / many-to-one8.4.2. One-to-one

8.5. Bidirectional associations with join tables

8.5.1. one-to-many / many-to-one8.5.2. one to one8.5.3. Many-to-many

8.6. More complex association mappings

9. Component Mapping

9.1. Dependent objects9.2. Collections of dependent objects9.3. Components as Map indices9.4. Components as composite identifiers9.5. Dynamic components

10. Inheritance mapping

10.1. The three strategies

10.1.1. Table per class hierarchy10.1.2. Table per subclass10.1.3. Table per subclass: using a discriminator10.1.4. Mixing table per class hierarchy with table per subclass10.1.5. Table per concrete class10.1.6. Table per concrete class using implicit polymorphism10.1.7. Mixing implicit polymorphism with other inheritance mappings

10.2. Limitations

11. Working with objects

11.1. Hibernate object states11.2. Making objects persistent11.3. Loading an object11.4. Querying

11.4.1. Executing queries11.4.2. Filtering collections11.4.3. Criteria queries11.4.4. Queries in native SQL

11.5. Modifying persistent objects11.6. Modifying detached objects11.7. Automatic state detection11.8. Deleting persistent objects11.9. Replicating object between two different datastores11.10. Flushing the Session11.11. Transitive persistence11.12. Using metadata

12. Read-only entities

12.1. Making persistent entities read-only

12.1.1. Entities of immutable classes12.1.2. Loading persistent entities as read-only12.1.3. Loading read-only entities from an HQL query/criteria12.1.4. Making a persistent entity read-only

12.2. Read-only affect on property type

12.2.1. Simple properties12.2.2. Unidirectional associations12.2.3. Bidirectional associations

13. Transactions and Concurrency

13.1. Session and transaction scopes

13.1.1. Unit of work13.1.2. Long conversations13.1.3. Considering object identity13.1.4. Common issues

13.2. Database transaction demarcation

13.2.1. Non-managed environment13.2.2. Using JTA13.2.3. Exception handling13.2.4. Transaction timeout

13.3. Optimistic concurrency control

13.3.1. Application version checking13.3.2. Extended session and automatic versioning13.3.3. Detached objects and automatic versioning13.3.4. Customizing automatic versioning

13.4. Pessimistic locking13.5. Connection release modes

14. Interceptors and events

14.1. Interceptors14.2. Event system14.3. Hibernate declarative security

15. Batch processing

15.1. Batch inserts

15.2. Batch updates15.3. The StatelessSession interface15.4. DML-style operations

16. HQL: The Hibernate Query Language

16.1. Case Sensitivity16.2. The from clause16.3. Associations and joins16.4. Forms of join syntax16.5. Referring to identifier property16.6. The select clause16.7. Aggregate functions16.8. Polymorphic queries16.9. The where clause16.10. Expressions16.11. The order by clause16.12. The group by clause16.13. Subqueries16.14. HQL examples16.15. Bulk update and delete16.16. Tips & Tricks16.17. Components16.18. Row value constructor syntax

17. Criteria Queries

17.1. Creating a Criteria instance

17.2. Narrowing the result set17.3. Ordering the results17.4. Associations17.5. Dynamic association fetching17.6. Components17.7. Collections17.8. Example queries17.9. Projections, aggregation and grouping17.10. Detached queries and subqueries17.11. Queries by natural identifier

18. Native SQL

18.1. Using a SQLQuery

18.1.1. Scalar queries18.1.2. Entity queries18.1.3. Handling associations and collections18.1.4. Returning multiple entities18.1.5. Returning non-managed entities18.1.6. Handling inheritance18.1.7. Parameters

18.2. Named SQL queries

18.2.1. Using return-property to explicitly specify column/alias names18.2.2. Using stored procedures for querying

18.3. Custom SQL for create, update and delete18.4. Custom SQL for loading

19. Filtering data

19.1. Hibernate filters

20. Improving performance

20.1. Fetching strategies

20.1.1. Working with lazy associations20.1.2. Tuning fetch strategies20.1.3. Single-ended association proxies20.1.4. Initializing collections and proxies20.1.5. Using batch fetching20.1.6. Using subselect fetching20.1.7. Fetch profiles20.1.8. Using lazy property fetching

20.2. The Second Level Cache

20.2.1. Cache mappings20.2.2. Strategy: read only20.2.3. Strategy: read/write20.2.4. Strategy: nonstrict read/write20.2.5. Strategy: transactional20.2.6. Cache-provider/concurrency-strategy compatibility

20.3. Managing the caches20.4. The Query Cache

20.4.1. Enabling query caching20.4.2. Query cache regions

20.5. Bytecode Enhancement

20.5.1. Implementing org.hibernate.engine.spi.ManagedEntity interface

20.5.2. Runtime instrument20.5.3. Buildtime instrument

20.6. Understanding Collection performance

20.6.1. Taxonomy20.6.2. Lists, maps, idbags and sets are the most efficient collections to update20.6.3. Bags and lists are the most efficient inverse collections20.6.4. One shot delete

20.7. Monitoring performance

20.7.1. Monitoring a SessionFactory20.7.2. Metrics

21. Toolset Guide

21.1. Automatic schema generation

21.1.1. Customizing the schema21.1.2. Running the tool21.1.3. Properties21.1.4. Using Ant21.1.5. Incremental schema updates21.1.6. Using Ant for incremental schema updates21.1.7. Schema validation21.1.8. Using Ant for schema validation

22. Additional modules

22.1. Bean Validation

22.1.1. Adding Bean Validation22.1.2. Configuration22.1.3. Catching violations22.1.4. Database schema

22.2. Hibernate Search

22.2.1. Description22.2.2. Integration with Hibernate Annotations

23. Example: Parent/Child

23.1. A note about collections23.2. Bidirectional one-to-many23.3. Cascading life cycle23.4. Cascades and unsaved-value

23.5. Conclusion

24. Example: Weblog Application

24.1. Persistent Classes24.2. Hibernate Mappings24.3. Hibernate Code

25. Example: Various Mappings

25.1. Employer/Employee25.2. Author/Work25.3. Customer/Order/Product25.4. Miscellaneous example mappings

25.4.1. "Typed" one-to-one association25.4.2. Composite key example25.4.3. Many-to-many with shared composite key attribute25.4.4. Content based discrimination25.4.5. Associations on alternate keys

26. Best Practices

27. Database Portability Considerations

27.1. Portability Basics27.2. Dialect27.3. Dialect resolution27.4. Identifier generation27.5. Database functions27.6. Type mappings

References

List of Tables

3.1. Hibernate JDBC Properties3.2. Hibernate Datasource Properties3.3. Hibernate Configuration Properties3.4. Hibernate JDBC and Connection Properties3.5. Hibernate Cache Properties3.6. Hibernate Transaction Properties3.7. Miscellaneous Properties3.8. Hibernate SQL Dialects (hibernate.dialect)

3.9. Hibernate Log Categories3.10. JTA TransactionManagers10.1. Features of inheritance mappings12.1. Affect of read-only entity on property types18.1. Alias injection names20.1. Cache Providers20.2. Cache Concurrency Strategy Support21.1. Summary21.2. SchemaExport Command Line Options

21.3. SchemaExport Connection Properties21.4. SchemaUpdate Command Line Options

21.5. SchemaValidator Command Line Options

List of Examples

4.1. Simple POJO representing a cat4.2. Disabling proxies in hbm.xml

4.3. Disabling proxies in annotations4.4. Proxying an interface in hbm.xml

4.5. Proxying an interface in annotations4.6. Specify custom tuplizers in annotations4.7. Specify custom tuplizers in hbm.xml

5.1. @NotFound annotation5.2. @OnDelete annotation5.3. @ForeignKey annotation5.4. One to One association6.1. Defining and registering the custom Type6.2. Defining the custom UserType6.3. Defining the custom CompositeUserType6.4. Overriding the standard StringType

6.5. Snippet from BasicType.java7.1. Hibernate uses its own collection implementations7.2. Collection mapping using @OneToMany and @JoinColumn7.3. Collection mapping using @OneToMany and @JoinTable7.4. Mapping a Set using <set>7.5. options of <one-to-many> element7.6. Elements of the <map> mapping7.7. Ordered lists using @OrderBy

7.8. Explicit index column using @OrderColumn

7.9. index-list element for indexed collections in xml mapping7.10. Use of target entity property as map key via @MapKey

7.11. Map key as basic type using @MapKeyColumn

7.12. map-key xml mapping element7.13. map-key-many-to-many7.14. Collection of basic types mapped via @ElementCollection

7.15. @ElementCollection for embeddable objects7.16. <element> tag for collection values using mapping files7.17. Sorted collection with @Sort7.18. Sorted collection using xml mapping7.19. Sorting in database using order-by7.20. Sorting via a query filter7.21. Bidirectional one to many with many to one side as association owner7.22. Bidirectional association with one to many side as owner7.23. Bidirectional one to many via Hibernate mapping files7.24. Many to many association via @ManyToMany7.25. Default values for @ManyToMany (uni-directional)

7.26. Default values for @ManyToMany (bi-directional)

7.27. Many to many association using Hibernate mapping files7.28. Effect of inverse vs. non-inverse side of many to many associations7.29. Bidirectional association with indexed collection7.30. Bidirectional association with indexed collection, but no index column7.31. Ternary association mapping7.32. Example classes Parent and Child

7.33. One to many unidirectional Parent-Child relationship using annotations

7.34. One to many unidirectional Parent-Child relationship using mapping files

7.35. Table definitions for unidirectional Parent-Child relationship

7.36. One to many bidirectional Parent-Child relationship using annotations

7.37. One to many bidirectional Parent-Child relationship using mapping files

7.38. Table definitions for bidirectional Parent-Child relationship

7.39. Enforcing NOT NULL constraint in unidirectional relation using annotations7.40. Enforcing NOT NULL constraint in unidirectional relation using mapping files7.41. Many to many Parent-Child relationship using annotations

7.42. Many to many Parent-Child relationship using mapping files

7.43. Table definitions for many to many releationship11.1. Defining a named query using @NamedQuery

11.2. Defining a named query using <query>

11.3. Parameter binding of a named query11.4. @OneToMany with orphanRemoval

18.1. Named sql query using the <sql-query> maping element18.2. Execution of a named query18.3. Named sql query with association18.4. Named query returning a scalar18.5. <resultset> mapping used to externalize mapping information18.6. Programmatically specifying the result mapping information18.7. Named SQL query using @NamedNativeQuery together with @SqlResultSetMapping

18.8. Implicit result set mapping18.9. Using dot notation in @FieldResult for specifying associations18.10. Scalar values via @ColumnResult

18.11. Custom CRUD via annotations18.12. Custom CRUD XML18.13. Overriding SQL statements for collections using annotations18.14. Overriding SQL statements for secondary tables18.15. Stored procedures and their return value19.1. @FilterDef and @Filter annotations19.2. Using @FilterJoinTable for filterting on the association table

19.3. @Filter annotation, disabling deduceAliasInjectionPoints19.4. Defining a filter definition via <filter-def>

19.5. Attaching a filter to a class or collection using <filter>

20.1. Specifying a fetch profile using @FetchProfile

20.2. Specifying a fetch profile using <fetch-profile> outside <class> node

20.3. Specifying a fetch profile using <fetch-profile> inside <class> node

20.4. Activating a fetch profile for a given Session

20.5. Definition of cache concurrency strategy via @Cache

20.6. Caching collections using annotations20.7. @Cache annotation with attributes

20.8. The Hibernate <cache> mapping element

20.9. Explcitly evicting a cached instance from the first level cache using Session.evict()

20.10. Second-level cache eviction via SessionFactoty.evict() and SessionFacyory.evictCollection()

20.11. Browsing the second-level cache entries via the Statistics API

20.12. Enabling Hibernate statistics22.1. Using custom groups for validation

Preface

Working with both Object-Oriented software and Relational Databases can be cumbersome and time consuming.Development costs are significantly higher due to a paradigm mismatch between how data is represented in objectsversus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The termObject/Relational Mapping refers to the technique of mapping data from an object model representation to a relationaldata model representation (and visa versa). See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.

Note

While having a strong background in SQL is not required to use Hibernate, having abasic understanding of the concepts can greatly help you understand Hibernate morefully and quickly. Probably the single best background is an understanding of datamodeling principles. You might want to consider these resources as a good startingpoint:

http://www.agiledata.org/essays/dataModeling101.html

http://en.wikipedia.org/wiki/Data_modeling

Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL datatypes), but also provides data query and retrieval facilities. It can significantly reduce development time otherwise spentwith manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of commondata persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQLand JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you andguarantees that your investment in relational technology and knowledge is as valid as always.

Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement thebusiness logic in the database, it is most useful with object-oriented domain models and business logic in the Java-basedmiddle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and will helpwith the common task of result set translation from a tabular representation to a graph of objects.

If you are new to Hibernate and Object/Relational Mapping or even Java, please follow these steps:

1. Read Chapter 1, Tutorial for a tutorial with step-by-step instructions. The source code for the tutorial is included inthe distribution in the doc/reference/tutorial/ directory.

2. Read Chapter 2, Architecture to understand the environments where Hibernate can be used.

3. View the eg/ directory in the Hibernate distribution. It contains a simple standalone application. Copy your JDBC

driver to the lib/ directory and edit etc/hibernate.properties, specifying correct values for your database. From a

command prompt in the distribution directory, type ant eg (using Ant), or under Windows, type build eg.

4. Use this reference documentation as your primary source of information. Consider reading [JPwH] if you needmore help with application design, or if you prefer a step-by-step tutorial. Also visithttp://caveatemptor.hibernate.org and download the example application from [JPwH].

5. FAQs are answered on the Hibernate website.

6. Links to third party demos, examples, and tutorials are maintained on the Hibernate website.

7. The Community Area on the Hibernate website is a good resource for design patterns and various integrationsolutions (Tomcat, JBoss AS, Struts, EJB, etc.).

There are a number of ways to become involved in the Hibernate community, including

Trying stuff out and reporting bugs. See http://hibernate.org/issuetracker.html details.

Trying your hand at fixing some bugs or implementing enhancements. Again, see http://hibernate.org/issuetracker.htmldetails.

http://hibernate.org/community.html lists a few ways to engage in the community.

There are forums for users to ask questions and receive help from the community.

There are also IRC channels for both user and developer discussions.

Helping improve or translate this documentation. Contact us on the developer mailing list if you have interest.

Evangelizing Hibernate within your organization.

Chapter 1. Tutorial

Table of Contents


1.1.1. Setup1.1.2. The first class1.1.3. The mapping file1.1.4. Hibernate configuration

http://en.wikipedia.org/wiki/Object-relational_mapping

http://www.agiledata.org/essays/dataModeling101.html

http://en.wikipedia.org/wiki/Data_modeling

http://caveatemptor.hibernate.org

http://hibernate.org/issuetracker.html

http://hibernate.org/issuetracker.html

http://hibernate.org/community.html

http://en.wikipedia.org/wiki/Internet_Relay_Chat

1.1.5. Building with Maven1.1.6. Startup and helpers1.1.7. Loading and storing objects


1.2.1. Mapping the Person class1.2.2. A unidirectional Set-based association1.2.3. Working the association1.2.4. Collection of values1.2.5. Bi-directional associations1.2.6. Working bi-directional links


1.3.1. Writing the basic servlet1.3.2. Processing and rendering1.3.3. Deploying and testing

1.4. Summary

Intended for new users, this chapter provides an step-by-step introduction to Hibernate, starting with a simple applicationusing an in-memory database. The tutorial is based on an earlier tutorial developed by Michael Gloegl. A l l code iscontained in the tutorials/web directory of the project source.

Important

This tutorial expects the user have knowledge of both Java and SQL. If you have a limitedknowledge of JAVA or SQL, it is advised that you start with a good introduction to thattechnology prior to attempting to learn Hibernate.

Note

The distribution contains another example application under t h e tutorial/eg project

source directory.


For this example, we will set up a small database application that can store events we want to attend and informationabout the host(s) of these events.

Note

Although you can use whatever database you feel comfortable using, we will useHSQLDB (an in-memory, Java database) to avoid describing installation/setup of anyparticular database servers.

1.1.1. Setup

The first thing we need to do is to set up the development environment. We will be using the "standard layout" advocatedby alot of build tools such as Maven. Maven, in particular, has a good resource describing this layout. As this tutorial is tobe a web application, we will be creating and making use of src/main/java, src/main/resources a n d src/main/webapp

directories.

We will be using Maven in this tutorial, taking advantage of its transitive dependency management capabilities as well asthe ability of many IDEs to automatically set up a project for us based on the maven descriptor.

http://hsqldb.org/

http://maven.apache.org

http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

<modelVersion>4.0.0</modelVersion>

<groupId>org.hibernate.tutorials</groupId> <artifactId>hibernate-tutorial</artifactId> <version>1.0.0-SNAPSHOT</version> <name>First Hibernate Tutorial</name>

<build>  <finalName>${artifactId}</finalName> </build>

<dependencies> <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-core</artifactId> </dependency>

 <dependency> <groupId>javax.servlet</groupId> <artifactId>servlet-api</artifactId> </dependency>

 <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-simple</artifactId> </dependency>

<dependency> <groupId>javassist</groupId> <artifactId>javassist</artifactId> </dependency> </dependencies>

</project>

Tip

It is not a requirement to use Maven. If you wish to use something else to build thistutorial (such as Ant), the layout will remain the same. The only change is that you willneed to manually account for all the needed dependencies. If you use something like Ivyproviding transitive dependency management you would still use the dependenciesmentioned below. Otherwise, you'd need to grab all dependencies, both explicit andtransitive, and add them to the project's classpath. If working from the Hibernatedistribution bundle, this would mean hibernate3.jar, all artifacts in the lib/required directory

and all files from either the lib/bytecode/cglib or lib/bytecode/javassist directory;

additionally you will need both the servlet-api jar and one of the slf4j logging backends.

Save this file as pom.xml in the project root directory.

1.1.2. The first class

Next, we create a class that represents the event we want to store in the database; it is a simple JavaBean class with someproperties:

package org.hibernate.tutorial.domain;

import java.util.Date;

public class Event { private Long id;

private String title; private Date date;

public Event() {}

public Long getId() { return id; }

private void setId(Long id) { this.id = id; }

public Date getDate() { return date; }

public void setDate(Date date) { this.date = date; }

public String getTitle() { return title; }

public void setTitle(String title) { this.title = title; }}

This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibilityfor the fields. Although this is the recommended design, it is not required. Hibernate can also access fields directly, thebenefit of accessor methods is robustness for refactoring.

The id property holds a unique identifier value for a particular event. All persistent entity classes (there are less important

dependent classes as well) will need such an identifier property if we want to use the full feature set of Hibernate. In fact,most applications, especially web applications, need to distinguish objects by identifier, so you should consider this afeature rather than a limitation. However, we usually do not manipulate the identity of an object, hence the setter methodshould be private. Only Hibernate will assign identifiers when an object is saved. Hibernate can access public, private,

http://ant.apache.org/ivy/

and protected accessor methods, as well as public, private and protected fields directly. The choice is up to you and youcan match it to fit your application design.

The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects for you, usingJava Reflection. The constructor can be private, however package or public visibility is required for runtime proxygeneration and efficient data retrieval without bytecode instrumentation.

Save this file to the src/main/java/org/hibernate/tutorial/domain directory.

1.1.3. The mapping file

Hibernate needs to know how to load and store objects of the persistent class. This is where the Hibernate mapping filecomes into play. The mapping file tells Hibernate what table in the database it has to access, and what columns in thattable it should use.

The basic structure of a mapping file looks like this:

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="org.hibernate.tutorial.domain">[...]</hibernate-mapping>

Hibernate DTD is sophisticated. You can use it for auto-completion of XML mapping elements and attributes in your editoror IDE. Opening up the DTD file in your text editor is the easiest way to get an overview of all elements and attributes, andto view the defaults, as well as some comments. Hibernate will not load the DTD file from the web, but first look it up fromthe classpath of the application. The DTD file is included in hibernate-core.jar (it is also included in the hibernate3.jar, if

using the distribution bundle).

Important

We will omit the DTD declaration in future examples to shorten the code. It is, of course,not optional.

Between the two hibernate-mapping tags, include a class element. All persistent entity classes (again, there might be

dependent classes later on, which are not first-class entities) need a mapping to a table in the SQL database:

<hibernate-mapping package="org.hibernate.tutorial.domain">

<class name="Event" table="EVENTS">

</class>

</hibernate-mapping>

So far we have told Hibernate how to persist and load object of class Event to the table EVENTS. Each instance is now

represented by a row in that table. Now we can continue by mapping the unique identifier property to the tables primarykey. As we do not want to care about handling this identifier, we configure Hibernate's identifier generation strategy for asurrogate primary key column:


<class name="Event" table="EVENTS"> <id name="id" column="EVENT_ID"> <generator class="native"/> </id> </class>


The id element is the declaration of the identifier property. The name="id" mapping attribute declares the name of the

JavaBean property and tells Hibernate to use the getId() and setId() methods to access the property. The column attribute

tells Hibernate which column of the EVENTS table holds the primary key value.

The nested generator element specifies the identifier generation strategy (aka how are identifier values generated?). In this

case we choose native, which offers a level of portability depending on the configured database dialect. Hibernate

supports database generated, globally unique, as well as application assigned, identifiers. Identifier value generation isalso one of Hibernate's many extension points and you can plugin in your own strategy.

Tip

native is no longer consider the best strategy in terms of portability. for further discussion,

see Section 27.4, “Identifier generation”

Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class areconsidered persistent:


<class name="Event" table="EVENTS"> <id name="id" column="EVENT_ID"> <generator class="native"/> </id> <property name="date" type="timestamp" column="EVENT_DATE"/> <property name="title"/> </class>


Similar to the id element, the name attribute of the property element tells Hibernate which getter and setter methods to use.

In this case, Hibernate will search for getDate(), setDate(), getTitle() and setTitle() methods.

Note

Why does the date property mapping include the column attribute, but the title does not?

Without the column attribute, Hibernate by default uses the property name as the column

name. This works for title, however, date is a reserved keyword in most databases so you

will need to map it to a different name.

The title mapping also lacks a type attribute. The types declared and used in the mapping files are not Java data types; they

are not SQL database types either. These types are called Hibernate mapping types, converters which can translate fromJava to SQL data types and vice versa. Again, Hibernate will try to determine the correct conversion and mapping typeitself if the type attribute is not present in the mapping. In some cases this automatic detection using Reflection on the Java

class might not have the default you expect or need. This is the case with the date property. Hibernate cannot know if the

property, which is of java.util.Date, should map to a SQL date, timestamp, or time column. Full date and time information is

preserved by mapping the property with a timestamp converter.

Tip

Hibernate makes this mapping type determination using reflection when the mappingfi les are processed. This can take time and resources, so if startup performance isimportant you should consider explicitly defining the type to use.

Save this mapping file as src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.

1.1.4. Hibernate configuration

At this point, you should have the persistent class and its mapping file in place. It is now time to configure Hibernate. Firstlet's set up HSQLDB to run in "server mode"

Note

We do this so that the data remains between runs.

We will utilize the Maven exec plugin to launch the HSQLDB server by running: mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial" You will see it start up andbind to a TCP/IP socket; this is where our application will connect later. If you want to start with a fresh database during thistutorial, shutdown HSQLDB, delete all files in the target/data directory, and start HSQLDB again.

Hibernate will be connecting to the database on behalf of your application, so it needs to know how to obtain connections.For this tutorial we will be using a standalone connection pool (as opposed to a javax.sql.DataSource). Hibernate comes

with support for two third-party open source JDBC connection pools: c3p0 and proxool. However, we will be using theHibernate built-in connection pool for this tutorial.

Caution

The built-in Hibernate connection pool is in no way intended for production use. It lacksseveral features found on any decent connection pool.

For Hibernate's configuration, we can use a simple hibernate.properties file, a more sophisticated hibernate.cfg.xml file, or

even complete programmatic setup. Most users prefer the XML configuration file:

<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

<session-factory>

 <property name="connection.driver_class">org.hsqldb.jdbcDriver</property> <property name="connection.url">jdbc:hsqldb:hsql://localhost</property> <property name="connection.username">sa</property> <property name="connection.password"></property>

 <property name="connection.pool_size">1</property>

 <property name="dialect">org.hibernate.dialect.HSQLDialect</property>

 <property name="current_session_context_class">thread</property>

 <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

 <property name="show_sql">true</property>

 <property name="hbm2ddl.auto">update</property>

<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>

</session-factory>

</hibernate-configuration>

Note

Notice that this configuration file specifies a different DTD

https://sourceforge.net/projects/c3p0

http://proxool.sourceforge.net/

You configure Hibernate's SessionFactory. SessionFactory is a global factory responsible for a particular database. If you

have several databases, for easier startup you should use several <session-factory> configurations in several configuration

files.

The first four property elements contain the necessary configuration for the JDBC connection. The dialect property element

specifies the particular SQL variant Hibernate generates.

Tip

In most cases, Hibernate is able to properly determine which dialect to use. SeeSection 27.3, “Dialect resolution” for more information.

Hibernate's automatic session management for persistence contexts is particularly useful in this context. The hbm2ddl.auto

option turns on automatic generation of database schemas directly into the database. This can also be turned off byremoving the configuration option, or redirected to a file with the help of the SchemaExport Ant task. Finally, add the

mapping file(s) for persistent classes to the configuration.

Save this file as hibernate.cfg.xml into the src/main/resources directory.

1.1.5. Building with Maven

We will now build the tutorial with Maven. You will need to have Maven installed; it is available from the Maven downloadpage. Maven will read the /pom.xml file we created earlier and know how to perform some basic project tasks. First, lets

run the compile goal to make sure we can compile everything so far:

[hibernateTutorial]$ mvn compile[INFO] Scanning for projects...[INFO] ------------------------------------------------------------------------[INFO] Building First Hibernate Tutorial[INFO] task-segment: [compile][INFO] ------------------------------------------------------------------------[INFO] [resources:resources][INFO] Using default encoding to copy filtered resources.[INFO] [compiler:compile][INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESSFUL[INFO] ------------------------------------------------------------------------[INFO] Total time: 2 seconds[INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009[INFO] Final Memory: 5M/547M[INFO] ------------------------------------------------------------------------

1.1.6. Startup and helpers

It is time to load and store some Event objects, but first you have to complete the setup with some infrastructure code. You

have to startup Hibernate by building a global org.hibernate.SessionFactory object and storing it somewhere for easy access

in application code. A org.hibernate.SessionFactory is used to obtain org.hibernate.Session instances. A org.hibernate.Session

represents a single-threaded unit of work. The org.hibernate.SessionFactory is a thread-safe global object that is instantiated

once.

We will create a HibernateUtil helper class that takes care of startup and makes accessing the org.hibernate.SessionFactory

more convenient.

package org.hibernate.tutorial.util;

import org.hibernate.SessionFactory;import org.hibernate.boot.registry.StandardServiceRegistryBuilder;import org.hibernate.cfg.Configuration;

public class HibernateUtil {

private static final SessionFactory sessionFactory = buildSessionFactory();

private static SessionFactory buildSessionFactory() { try { // Create the SessionFactory from hibernate.cfg.xml new Configuration().configure().buildSessionFactory( new StandardServiceRegistryBuilder().build() ); } catch (Throwable ex) { // Make sure you log the exception, as it might be swallowed System.err.println("Initial SessionFactory creation failed." + ex); throw new ExceptionInInitializerError(ex); } }

public static SessionFactory getSessionFactory() { return sessionFactory; }

}

Save this code as src/main/java/org/hibernate/tutorial/util/HibernateUtil.java

This class not only produces the global org.hibernate.SessionFactory reference in its static initializer; it also hides the fact that

it uses a static singleton. We might just as well have looked up the org.hibernate.SessionFactory reference from JNDI in an

application server or any other location for that matter.

If you give the org.hibernate.SessionFactory a name in your configuration, Hibernate will try to bind it to JNDI under that

name after it has been built. Another, better option is to use a JMX deployment and let the JMX-capable containerinstantiate and bind a HibernateService to JNDI. Such advanced options are discussed later.

You now need to configure a logging system. Hibernate uses commons logging and provides two choices: Log4j and JDK1.4 logging. Most developers prefer Log4j: copy log4j.properties from the Hibernate distribution in the etc/ directory to your

http://maven.apache.org/download.html

src directory, next to hibernate.cfg.xml. If you prefer to have more verbose output than that provided in the example

configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.

The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.

1.1.7. Loading and storing objects

We are now ready to start doing some real work with Hibernate. Let's start by writing an EventManager class with a main()

method:

package org.hibernate.tutorial;

import org.hibernate.Session;

import java.util.*;

import org.hibernate.tutorial.domain.Event;import org.hibernate.tutorial.util.HibernateUtil;

public class EventManager {

public static void main(String[] args) { EventManager mgr = new EventManager();

if (args[0].equals("store")) { mgr.createAndStoreEvent("My Event", new Date()); }

HibernateUtil.getSessionFactory().close(); }

private void createAndStoreEvent(String title, Date theDate) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction();

Event theEvent = new Event(); theEvent.setTitle(title); theEvent.setDate(theDate); session.save(theEvent);

session.getTransaction().commit(); }

}

In createAndStoreEvent() we created a new Event object and handed it over to Hibernate. At that point, Hibernate takes care

of the SQL and executes an INSERT on the database.

A org.hibernate.Session is designed to represent a single unit of work (a single atomic piece of work to be performed). For

now we will keep things simple and assume a one-to-one granularity between a Hibernate org.hibernate.Session and a

database transaction. To shield our code from the actual underlying transaction system we use the Hibernateorg.hibernate.Transaction API. In this particular case we are using JDBC-based transactional semantics, but it could also run

with JTA.

What does sessionFactory.getCurrentSession() do? First, you can call it as many times and anywhere you like once you get

hold of your org.hibernate.SessionFactory. T h e getCurrentSession() method always returns the "current" unit of work.

Remember that we switched the configuration option for this mechanism to "thread" in oursrc/main/resources/hibernate.cfg.xml? Due to that setting, the context of a current unit of work is bound to the current Java

thread that executes the application.

Important

Hibernate offers three methods of current session tracking. The "thread" based method isnot intended for production use; it is merely useful for prototyping and tutorials such asthis one. Current session tracking is discussed in more detail later on.

A org.hibernate.Session begins when the first call to getCurrentSession() is made for the current thread. It is then bound by

Hibernate to the current thread. When the transaction ends, either through commit or rollback, Hibernate automaticallyunbinds the org.hibernate.Session from the thread and closes it for you. If you call getCurrentSession() again, you get a new

org.hibernate.Session and can start a new unit of work.

Related to the unit of work scope, should the Hibernate org.hibernate.Session be used to execute one or several database

operations? The above example uses one org.hibernate.Session for one operation. However this is pure coincidence; the

example is just not complex enough to show any other approach. The scope of a Hibernate org.hibernate.Session is flexible

but you should never design your application to use a new Hibernate org.hibernate.Session for every database operation.

Even though it is used in the following examples, consider session-per-operation an anti-pattern. A real web application isshown later in the tutorial which will help illustrate this.

See Chapter 13, Transactions and Concurrency for more information about transaction handling and demarcation. Theprevious example also skipped any error handling and rollback.

To run this, we will make use of the Maven exec plugin to call our class with the necessary classpath setup: mvnexec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="store"

Note

You may need to perform mvn compile first.

You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards the end, thefollowing line will be displayed:

[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)

This is the INSERT executed by Hibernate.

To list stored events an option is added to the main method:

if (args[0].equals("store")) { mgr.createAndStoreEvent("My Event", new Date()); } else if (args[0].equals("list")) { List events = mgr.listEvents(); for (int i = 0; i < events.size(); i++) { Event theEvent = (Event) events.get(i); System.out.println( "Event: " + theEvent.getTitle() + " Time: " + theEvent.getDate() ); } }

A new listEvents() method is also added:

private List listEvents() { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); List result = session.createQuery("from Event").list(); session.getTransaction().commit(); return result; }

Here, we are using a Hibernate Query Language (HQL) query to load all existing Event objects from the database.

Hibernate will generate the appropriate SQL, send it to the database and populate Event objects with the data. You can

create more complex queries with HQL. See Chapter 16, HQL: The Hibernate Query Language for more information.

Now we can call our new functionality, again using the Maven exec plugin: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"


So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit and add someclass associations. We will add people to the application and store a list of events in which they participate.

1.2.1. Mapping the Person class

The first cut of the Person class looks like this:

package org.hibernate.tutorial.domain;

public class Person {

private Long id; private int age; private String firstname; private String lastname;

public Person() {}

// Accessor methods for all properties, private setter for 'id'

}

Save this to a file named src/main/java/org/hibernate/tutorial/domain/Person.java

Next, create the new mapping file as src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml


<class name="Person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="native"/> </id> <property name="age"/> <property name="firstname"/> <property name="lastname"/> </class>


Finally, add the new mapping to Hibernate's configuration:

<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/><mapping resource="org/hibernate/tutorial/domain/Person.hbm.xml"/>

Create an association between these two entities. Persons can participate in events, and events have participants. Thedesign questions you have to deal with are: directionality, multiplicity, and collection behavior.

1.2.2. A unidirectional Set-based association

By adding a collection of events to the Person class, you can easily navigate to the events for a particular person, without

executing an explicit query - by calling Person#getEvents. Multi-valued associations are represented in Hibernate by one of

the Java Collection Framework contracts; here we choose a java.util.Set because the collection will not contain duplicate

elements and the ordering is not relevant to our examples:

public class Person {

private Set events = new HashSet();

public Set getEvents() { return events; }

public void setEvents(Set events) { this.events = events; }}

Before mapping this association, let's consider the other side. We could just keep this unidirectional or create anothercollection on the Event, if we wanted to be able to navigate it from both directions. This is not necessary, from a functional

perspective. You can always execute an explicit query to retrieve the participants for a particular event. This is a designchoice left to you, but what is clear from this discussion is the multiplicity of the association: "many" valued on both sides iscalled a many-to-many association. Hence, we use Hibernate's many-to-many mapping:

<class name="Person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="native"/> </id> <property name="age"/> <property name="firstname"/> <property name="lastname"/>

<set name="events" table="PERSON_EVENT"> <key column="PERSON_ID"/> <many-to-many column="EVENT_ID" class="Event"/> </set>

</class>

Hibernate supports a broad range of collection mappings, a set being most common. For a many-to-many association, or

n:m entity relationship, an association table is required. Each row in this table represents a link between a person and anevent. The table name is declared using the table attribute of the set element. The identifier column name in the

association, for the person side, is defined with the key element, the column name for the event's side with the column

attribute of the many-to-many. You also have to tell Hibernate the class of the objects in your collection (the class on the

other side of the collection of references).

The database schema for this mapping is therefore:

_____________ __________________ | | | | _____________ | EVENTS | | PERSON_EVENT | | | |_____________| |__________________| | PERSON | | | | | |_____________| | *EVENT_ID | <--> | *EVENT_ID | | | | EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | | TITLE | |__________________| | AGE | |_____________| | FIRSTNAME | | LASTNAME | |_____________|

1.2.3. Working the association

Now we will bring some people and events together in a new method in EventManager:

private void addPersonToEvent(Long personId, Long eventId) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction();

Person aPerson = (Person) session.load(Person.class, personId); Event anEvent = (Event) session.load(Event.class, eventId); aPerson.getEvents().add(anEvent);


After loading a Person and an Event, simply modify the collection using the normal collection methods. There is no explicit

call to update() or save(); Hibernate automatically detects that the collection has been modified and needs to be updated.

This is called automatic dirty checking. You can also try it by modifying the name or the date property of any of yourobjects. As long as they are in persistent state, that is, bound to a particular Hibernate org.hibernate.Session, Hibernate

monitors any changes and executes SQL in a write-behind fashion. The process of synchronizing the memory state withthe database, usually only at the end of a unit of work, is called flushing. In our code, the unit of work ends with a commit,or rollback, of the database transaction.

You can load person and event in different units of work. Or you can modify an object outside of a org.hibernate.Session,

when it is not in persistent state (if it was persistent before, this state is called detached). You can even modify a collectionwhen it is detached:

private void addPersonToEvent(Long personId, Long eventId) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction();

Person aPerson = (Person) session .createQuery("select p from Person p left join fetch p.events where p.id = :pid") .setParameter("pid", personId) .uniqueResult(); // Eager fetch the collection so we can use it detached Event anEvent = (Event) session.load(Event.class, eventId);

session.getTransaction().commit();

// End of first unit of work

aPerson.getEvents().add(anEvent); // aPerson (and its collection) is detached

// Begin second unit of work

Session session2 = HibernateUtil.getSessionFactory().getCurrentSession(); session2.beginTransaction(); session2.update(aPerson); // Reattachment of aPerson

session2.getTransaction().commit(); }

The call to update makes a detached object persistent again by binding it to a new unit of work, so any modifications you

made to it while detached can be saved to the database. This includes any modifications (additions/deletions) you madeto a collection of that entity object.

This is not much use in our example, but it is an important concept you can incorporate into your own application.Complete this exercise by adding a new action to the main method of the EventManager and call it from the command line.

If you need the identifiers of a person and an event - the save() method returns it (you might have to modify some of the

previous methods to return that identifier):

else if (args[0].equals("addpersontoevent")) { Long eventId = mgr.createAndStoreEvent("My Event", new Date()); Long personId = mgr.createAndStorePerson("Foo", "Bar"); mgr.addPersonToEvent(personId, eventId); System.out.println("Added person " + personId + " to event " + eventId); }

This is an example of an association between two equally important classes : two entities. As mentioned earlier, there areother classes and types in a typical model, usually "less important". Some you have already seen, like an int or a

java.lang.String. We call these classes value types, and their instances depend on a particular entity. Instances of these

types do not have their own identity, nor are they shared between entities. Two persons do not reference the samefirstname object, even if they have the same first name. Value types cannot only be found in the JDK , but you can also

write dependent classes yourself such as an Address or MonetaryAmount class. In fact, in a Hibernate application all JDK

classes are considered value types.

You can also design a collection of value types. This is conceptually different from a collection of references to otherentities, but looks almost the same in Java.

1.2.4. Collection of values

Let's add a collection of email addresses to the Person entity. This will be represented as a java.util.Set of java.lang.String

instances:

private Set emailAddresses = new HashSet();

public Set getEmailAddresses() { return emailAddresses; }

public void setEmailAddresses(Set emailAddresses) { this.emailAddresses = emailAddresses; }

The mapping of this Set is as follows:

<set name="emailAddresses" table="PERSON_EMAIL_ADDR"> <key column="PERSON_ID"/> <element type="string" column="EMAIL_ADDR"/> </set>

The difference compared with the earlier mapping is the use of the element part which tells Hibernate that the collection

does not contain references to another entity, but is rather a collection whose elements are values types, here specificallyof type string. The lowercase name tells you it is a Hibernate mapping type/converter. Again the table attribute of the set

element determines the table name for the collection. The key element defines the foreign-key column name in the

collection table. The column attribute in the element element defines the column name where the email address values will

actually be stored.

Here is the updated schema:

_____________ __________________ | | | | _____________ | EVENTS | | PERSON_EVENT | | | ___________________ |_____________| |__________________| | PERSON | | | | | | | |_____________| | PERSON_EMAIL_ADDR | | *EVENT_ID | <--> | *EVENT_ID | | | |___________________| | EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | <--> | *PERSON_ID | | TITLE | |__________________| | AGE | | *EMAIL_ADDR | |_____________| | FIRSTNAME | |___________________| | LASTNAME | |_____________|

You can see that the primary key of the collection table is in fact a composite key that uses both columns. This also impliesthat there cannot be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.

You can now try to add elements to this collection, just like we did before by linking persons and events. It is the samecode in Java:

private void addEmailToPerson(Long personId, String emailAddress) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction();

Person aPerson = (Person) session.load(Person.class, personId); // adding to the emailAddress collection might trigger a lazy load of the collection aPerson.getEmailAddresses().add(emailAddress);


This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize this with an eagerfetch.

1.2.5. Bi-directional associations

Next you will map a bi-directional association. You will make the association between person and event work from bothsides in Java. The database schema does not change, so you will still have many-to-many multiplicity.

Note

A relational database is more flexible than a network programming language, in that itdoes not need a navigation direction; data can be viewed and retrieved in any possibleway.

First, add a collection of participants to the Event class:

private Set participants = new HashSet();

public Set getParticipants() { return participants; }

public void setParticipants(Set participants) { this.participants = participants; }

Now map this side of the association in Event.hbm.xml.

<set name="participants" table="PERSON_EVENT" inverse="true"> <key column="EVENT_ID"/> <many-to-many column="PERSON_ID" class="Person"/> </set>

These are normal set mappings in both mapping documents. Notice that the column names in key and many-to-many swap

in both mapping documents. The most important addition here is the inverse="true" attribute in the set element of the Event's

collection mapping.

What this means is that Hibernate should take the other side, the Person class, when it needs to find out information about

the link between the two. This will be a lot easier to understand once you see how the bi-directional link between our twoentities is created.

1.2.6. Working bi-directional links

First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a link between a Person and

an Event in the unidirectional example? You add an instance of Event to the collection of event references, of an instance

o f Person. If you want to make this link bi-directional, you have to do the same on the other side by adding a Person

reference to the collection in an Event. This process of "setting the link on both sides" is absolutely necessary with bi-

directional links.

Many developers program defensively and create link management methods to correctly set both sides (for example, inPerson):

protected Set getEvents() { return events; }

protected void setEvents(Set events) { this.events = events; }

public void addToEvent(Event event) { this.getEvents().add(event); event.getParticipants().add(this); }

public void removeFromEvent(Event event) { this.getEvents().remove(event); event.getParticipants().remove(this); }

The get and set methods for the collection are now protected. This allows classes in the same package and subclasses tostill access the methods, but prevents everybody else from altering the collections directly. Repeat the steps for thecollection on the other side.

What about the inverse mapping attribute? For you, and for Java, a bi-directional link is simply a matter of setting the

references on both sides correctly. Hibernate, however, does not have enough information to correctly arrange SQLINSERT and UPDATE statements (to avoid constraint violations). Making one side of the association inverse tells Hibernate

to consider it a mirror of the other side. That is all that is necessary for Hibernate to resolve any issues that arise whentransforming a directional navigation model to a SQL database schema. The rules are straightforward: all bi-directionalassociations need one side as inverse. In a one-to-many association it has to be the many-side, and in many-to-many

association you can select either side.


A Hibernate web application uses Session and Transaction almost like a standalone application. However, some common

patterns are useful. You can now write an EventManagerServlet. This servlet can list all events stored in the database, and it

provides an HTML form to enter new events.

1.3.1. Writing the basic servlet

First we need create our basic processing servlet. Since our servlet only handles HTTP GET requests, we will only

implement the doGet() method:

package org.hibernate.tutorial.web;

// Imports

public class EventManagerServlet extends HttpServlet {

protected void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" );

try { // Begin unit of work HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();

// Process request and render page...

// End unit of work HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit(); } catch (Exception ex) { HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback(); if ( ServletException.class.isInstance( ex ) ) { throw ( ServletException ) ex; } else { throw new ServletException( ex ); } } }

}

Save this servlet as src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java

The pattern applied here is called session-per-request. When a request hits the servlet, a new Hibernate Session is

opened through the first call to getCurrentSession() on the SessionFactory. A database transaction is then started. All data

access occurs inside a transaction irrespective of whether the data is read or written. Do not use the auto-commit mode inapplications.

Do not use a new Hibernate Session for every database operation. Use one Hibernate Session that is scoped to the whole

request. Use getCurrentSession(), so that it is automatically bound to the current Java thread.

Next, the possible actions of the request are processed and the response HTML is rendered. We will get to that part soon.

Finally, the unit of work ends when processing and rendering are complete. If any problems occurred during processing orrendering, an exception will be thrown and the database transaction rolled back. This completes the session-per-request

pattern. Instead of the transaction demarcation code in every servlet, you could also write a servlet filter. See the Hibernatewebsite and Wiki for more information about this pattern called Open Session in View. You will need it as soon as youconsider rendering your view in JSP, not in a servlet.

1.3.2. Processing and rendering

Now you can implement the processing of the request and the rendering of the page.

// Write HTML header PrintWriter out = response.getWriter(); out.println("<html><head><title>Event Manager</title></head><body>");

// Handle actions if ( "store".equals(request.getParameter("action")) ) {

String eventTitle = request.getParameter("eventTitle"); String eventDate = request.getParameter("eventDate");

if ( "".equals(eventTitle) || "".equals(eventDate) ) { out.println("<b><i>Please enter event title and date.</i></b>"); } else { createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate)); out.println("<b><i>Added event.</i></b>"); } }

// Print page printEventForm(out); listEvents(out, dateFormatter);

// Write HTML footer out.println("</body></html>"); out.flush(); out.close();

This coding style, with a mix of Java and HTML, would not scale in a more complex application;keep in mind that we areonly illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page,an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputsHTML:

private void printEventForm(PrintWriter out) { out.println("<h2>Add new event:</h2>"); out.println("<form>"); out.println("Title: <input name='eventTitle' length='50'/><br/>"); out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>"); out.println("<input type='submit' name='action' value='store'/>"); out.println("</form>"); }

The listEvents() method uses the Hibernate Session bound to the current thread to execute a query:

private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) {

List result = HibernateUtil.getSessionFactory() .getCurrentSession().createCriteria(Event.class).list(); if (result.size() > 0) { out.println("<h2>Events in database:</h2>"); out.println("<table border='1'>"); out.println("<tr>"); out.println("<th>Event title</th>"); out.println("<th>Event date</th>"); out.println("</tr>"); Iterator it = result.iterator(); while (it.hasNext()) { Event event = (Event) it.next(); out.println("<tr>"); out.println("<td>" + event.getTitle() + "</td>"); out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>"); out.println("</tr>"); } out.println("</table>"); } }

Finally, the store action is dispatched to the createAndStoreEvent() method, which also uses the Session of the current thread:

protected void createAndStoreEvent(String title, Date theDate) { Event theEvent = new Event(); theEvent.setTitle(title); theEvent.setDate(theDate);

HibernateUtil.getSessionFactory() .getCurrentSession().save(theEvent); }

The servlet is now complete. A request to the servlet will be processed in a single Session and Transaction. As earlier in the

standalone application, Hibernate can automatically bind these objects to the current thread of execution. This gives youthe freedom to layer your code and access the SessionFactory in any way you like. Usually you would use a more

sophisticated design and move the data access code into data access objects (the DAO pattern). See the Hibernate Wikifor more examples.

1.3.3. Deploying and testing

To deploy this application for testing we must create a Web ARchive (WAR). First we must define the WAR descriptor assrc/main/webapp/WEB-INF/web.xml

<?xml version="1.0" encoding="UTF-8"?><web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

<servlet> <servlet-name>Event Manager</servlet-name> <servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class> </servlet>

<servlet-mapping> <servlet-name>Event Manager</servlet-name> <url-pattern>/eventmanager</url-pattern> </servlet-mapping></web-app>

To build and deploy call mvn package in your project directory and copy the hibernate-tutorial.war file into your Tomcat

webapps directory.

Note

If you do not have Tomcat installed, download it from http://tomcat.apache.org/ and followthe installation instructions. Our application requires no changes to the standard Tomcatconfiguration.

Once deployed and Tomcat is running, access the application at http://localhost:8080/hibernate-tutorial/eventmanager. Make

sure you watch the Tomcat log to see Hibernate initialize when the first request hits your servlet (the static initializer inHibernateUtil is called) and to get the detailed output if any exceptions occurs.

1.4. Summary

This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application. Moretutorials are available from the Hibernate website.

Chapter 2. Architecture

Table of Contents

2.1. Overview

2.1.1. Minimal architecture2.1.2. Comprehensive architecture2.1.3. Basic APIs


http://tomcat.apache.org/

http://hibernate.org

2.1. Overview

The diagram below provides a high-level view of the Hibernate architecture:

Unfortunately we cannot provide a detailed view of all possible runtime architectures. Hibernate is sufficiently flexible to beused in a number of ways in many, many architectures. We will, however, illustrate 2 specifically since they are extremes.

2.1.1. Minimal architecture

The "minimal" architecture has the application manage its own JDBC connections and provide those connections toHibernate; additionally the application manages transactions for itself. This approach uses a minimal subset of HibernateAPIs.

2.1.2. Comprehensive architecture

The "comprehensive" architecture abstracts the application away from the underlying JDBC/JTA APIs and allowsHibernate to manage the details.

2.1.3. Basic APIs

Here are quick discussions about some of the API objects depicted in the preceding diagrams (you will see them again inmore detail in later chapters).

SessionFactory (org.hibernate.SessionFactory)

A thread-safe, immutable cache of compiled mappings for a single database. A factory for org.hibernate.Session

instances. A client of org.hibernate.connection.ConnectionProvider. Optionally maintains a second level cache of data that is

reusable between transactions at a process or cluster level.

Session (org.hibernate.Session)

A single-threaded, short-lived object representing a conversation between the application and the persistent store.Wraps a JDBC java.sql.Connection. Factory for org.hibernate.Transaction. Maintains a first level cache of persistent the

application's persistent objects and collections; this cache is used when navigating the object graph or looking upobjects by identifier.

Persistent objects and collections

Short-lived, single threaded objects containing persistent state and business function. These can be ordinaryJavaBeans/POJOs. They are associated with exactly one org.hibernate.Session. Once the org.hibernate.Session is closed,

they will be detached and free to use in any application layer (for example, directly as data transfer objects to andfrom presentation). Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transient and detached objects and collections

Instances of persistent classes that are not currently associated with a org.hibernate.Session. They may have been

instantiated by the application and not yet persisted, or they may have been instantiated by a closedorg.hibernate.Session. Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transaction (org.hibernate.Transaction)

(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts theapplication from the underlying JDBC, JTA or CORBA transaction. A org.hibernate.Session might span several

org.hibernate.Transactions in some cases. However, transaction demarcation, either using the underlying API or

org.hibernate.Transaction, is never optional.

ConnectionProvider (org.hibernate.connection.ConnectionProvider)

(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlyingjavax.sql.DataSource or java.sql.DriverManager. It is not exposed to application, but it can be extended and/or

implemented by the developer.

TransactionFactory (org.hibernate.TransactionFactory)

(Optional) A factory for org.hibernate.Transaction instances. It is not exposed to the application, but it can be extended

and/or implemented by the developer.

Extension Interfaces

Hibernate offers a range of optional extension interfaces you can implement to customize the behavior of yourpersistence layer. See the API documentation for details.


Most applications using Hibernate need some form of "contextual" session, where a given session is in effect throughoutthe scope of a given context. However, across applications the definition of what constitutes a context is typically different;different contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tendedto utilize either home-grown ThreadLocal-based contextual sessions, helper classes such as HibernateUtil, or utilized third-

party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession() method. Initially, this assumed usage of

JTA transactions, where the JTA transaction defined both the scope and context of a current session. Given the maturity of

the numerous stand-alone JTA TransactionManager implementations, most, if not all, applications should be using JTA

transaction management, whether or not they are deployed into a J2EE container. Based on that, the JTA-based contextual

sessions are all you need to use.

However, as of version 3.1, the processing behind SessionFactory.getCurrentSession() is now pluggable. To that end, a new

extension interface, org.hibernate.context.spi.CurrentSessionContext, and a new configuration parameter,

hibernate.current_session_context_class, have been added to allow pluggability of the scope and context of defining current

sessions.

See the Javadocs for the org.hibernate.context.spi.CurrentSessionContext interface for a detailed discussion of its contract. It

defines a single method, currentSession(), by which the implementation is responsible for tracking the current contextual

session. Out-of-the-box, Hibernate comes with three implementations of this interface:

org.hibernate.context.internal.JTASessionContext: current sessions are tracked and scoped by a JTA transaction. The

processing here is exactly the same as in the older JTA-only approach. See the Javadocs for details.

org.hibernate.context.internal.ThreadLocalSessionContext:current sessions are tracked by thread of execution. See the

Javadocs for details.

org.hibernate.context.internal.ManagedSessionContext: current sessions are tracked by thread of execution. However, you

are responsible to bind and unbind a Session instance with static methods on this class: it does not open, flush, or

close a Session.

The first two implementations provide a "one session - one database transaction" programming model. This is also knownand used as session-per-request. The beginning and end of a Hibernate session is defined by the duration of a databasetransaction. If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to use theHibernate Transaction API to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA

interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries aredefined declaratively and you do not need any transaction or session demarcation operations in your code. Refer toChapter 13, Transactions and Concurrency for more information and code examples.

T h e hibernate.current_session_context_class configuration parameter defines which

org.hibernate.context.spi.CurrentSessionContext implementation should be used. For backwards compatibility, if this

configuration parameter is not set but a org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform is configured, Hibernate will

use the org.hibernate.context.internal.JTASessionContext. Typically, the value of this parameter would just name the

implementation class to use. For the three out-of-the-box implementations, however, there are three corresponding shortnames: "jta", "thread", and "managed".

Chapter 3. Configuration

Table of Contents

3.1. Programmatic configuration3.2. Obtaining a SessionFactory3.3. JDBC connections3.4. Optional configuration properties

3.4.1. SQL Dialects3.4.2. Outer Join Fetching3.4.3. Binary Streams3.4.4. Second-level and query cache3.4.5. Query Language Substitution3.4.6. Hibernate statistics

3.5. Logging3.6. Implementing a NamingStrategy

3.7. Implementing a PersisterClassProvider3.8. XML configuration file3.9. Java EE Application Server integration

3.9.1. Transaction strategy configuration3.9.2. JNDI-bound SessionFactory


Hibernate is designed to operate in many different environments and, as such, there is a broad range of configurationparameters. Fortunately, most have sensible default values and Hibernate is distributed with an examplehibernate.properties file in etc/ that displays the various options. Simply put the example file in your classpath and customize

it to suit your needs.

3.1. Programmatic configuration

An instance of org.hibernate.cfg.Configuration represents an entire set of mappings of an application's Java types to an SQL

database. The org.hibernate.cfg.Configuration is used to build an immutable org.hibernate.SessionFactory. The mappings are

compiled from various XML mapping files.

You can obtain a org.hibernate.cfg.Configuration instance by instantiating it directly and specifying XML mapping documents.

If the mapping files are in the classpath, use addResource(). For example:

Configuration cfg = new Configuration() .addResource("Item.hbm.xml") .addResource("Bid.hbm.xml");

An alternative way is to specify the mapped class and allow Hibernate to find the mapping document for you:

Configuration cfg = new Configuration() .addClass(org.hibernate.auction.Item.class) .addClass(org.hibernate.auction.Bid.class);

Hibernate will then search for mapping files named /org/hibernate/auction/Item.hbm.xml and

/org/hibernate/auction/Bid.hbm.xml in the classpath. This approach eliminates any hardcoded filenames.

A org.hibernate.cfg.Configuration also allows you to specify configuration properties. For example:

Configuration cfg = new Configuration() .addClass(org.hibernate.auction.Item.class) .addClass(org.hibernate.auction.Bid.class) .setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLInnoDBDialect") .setProperty("hibernate.connection.datasource", "java:comp/env/jdbc/test") .setProperty("hibernate.order_updates", "true");

This is not the only way to pass configuration properties to Hibernate. Some alternative options include:

1. Pass an instance of java.util.Properties to Configuration.setProperties().

2. Place a file named hibernate.properties in a root directory of the classpath.

3. Set System properties using java -Dproperty=value.

4. Include <property> elements in hibernate.cfg.xml (this is discussed later).

If you want to get started quicklyhibernate.properties is the easiest approach.

Th e org.hibernate.cfg.Configuration is intended as a startup-time object that will be discarded once a SessionFactory is

created.

3.2. Obtaining a SessionFactory

When all mappings have been parsed by the org.hibernate.cfg.Configuration, the application must obtain a factory for

org.hibernate.Session instances. This factory is intended to be shared by all application threads:

SessionFactory sessions = cfg.buildSessionFactory();

Hibernate does allow your application to instantiate more than one org.hibernate.SessionFactory. This is useful if you are

using more than one database.

3.3. JDBC connections

It is advisable to have the org.hibernate.SessionFactory create and pool JDBC connections for you. If you take this approach,

opening a org.hibernate.Session is as simple as:

Session session = sessions.openSession(); // open a new Session

Once you start a task that requires access to the database, a JDBC connection will be obtained from the pool.

Before you can do this, you first need to pass some JDBC connection properties to Hibernate. All Hibernate propertynames and semantics are defined on the class org.hibernate.cfg.Environment. The most important settings for JDBC

connection configuration are outlined below.

Hibernate will obtain and pool connections using java.sql.DriverManager if you set the following properties:

Table 3.1. Hibernate JDBC Properties

Property name Purpose

hibernate.connection.driver_class JDBC driver class

hibernate.connection.url JDBC URL

hibernate.connection.username database user

hibernate.connection.password database user password

hibernate.connection.pool_size maximum number of pooled connections

Hibernate's own connection pooling algorithm is, however, quite rudimentary. It is intended to help you get started and isnot intended for use in a production system, or even for performance testing. You should use a third party pool for bestperformance and stability. Just replace the hibernate.connection.pool_size property with connection pool specific settings.This will turn off Hibernate's internal pool. For example, you might like to use c3p0.

C3P0 is an open source JDBC connection pool distributed along with Hibernate in the lib directory. Hibernate will use its

org.hibernate.connection.C3P0ConnectionProvider for connection pooling if you set hibernate.c3p0.* properties. If you would

like to use Proxool, refer to the packaged hibernate.properties and the Hibernate web site for more information.

The following is an example hibernate.properties file for c3p0:

hibernate.connection.driver_class = org.postgresql.Driverhibernate.connection.url = jdbc:postgresql://localhost/mydatabasehibernate.connection.username = myuserhibernate.connection.password = secrethibernate.c3p0.min_size=5hibernate.c3p0.max_size=20hibernate.c3p0.timeout=1800hibernate.c3p0.max_statements=50hibernate.dialect = org.hibernate.dialect.PostgreSQL82Dialect

For use inside an application server, you should almost always configure Hibernate to obtain connections from anapplication server javax.sql.Datasource registered in JNDI. You will need to set at least one of the following properties:

Table 3.2. Hibernate Datasource Properties


hibernate.connection.datasource datasource JNDI name

hibernate.jndi.url URL of the JNDI provider (optional)

hibernate.jndi.class class of the JNDI InitialContextFactory (optional)

hibernate.connection.username database user (optional)

hibernate.connection.password database user password (optional)

Here is an example hibernate.properties file for an application server provided JNDI datasource:

hibernate.connection.datasource = java:/comp/env/jdbc/testhibernate.transaction.factory_class = \ org.hibernate.transaction.JTATransactionFactoryhibernate.transaction.manager_lookup_class = \ org.hibernate.transaction.JBossTransactionManagerLookuphibernate.dialect = org.hibernate.dialect.PostgreSQL82Dialect

JDBC connections obtained from a JNDI datasource will automatically participate in the container-managed transactionsof the application server.

Arbitrary connection properties can be given by prepending "hibernate.connection" to the connection property name. For

example, you can specify a charSet connection property using hibernate.connection.charSet.

You can define your own plugin strategy for obtaining JDBC connections by implementing the interfaceorg.hibernate.connection.ConnectionProvider, and specifying your custom implementation via the

hibernate.connection.provider_class property.

3.4. Optional configuration properties

There are a number of other properties that control the behavior of Hibernate at runtime. All are optional and havereasonable default values.

Warning

Some of these properties are "system-level" only. System-level properties can be set only

v i a java -Dproperty=value or hibernate.properties. They cannot be set by the other

techniques described above.

Table 3.3. Hibernate Configuration Properties


hibernate.dialect

The classname of a Hibernate org.hibernate.dialect.Dialect which allows

Hibernate to generate SQL optimized for a particular relational database.

e.g. full.classname.of.Dialect

I n most cases Hibernate will actually be able to choose the correctorg.hibernate.dialect.Dialect implementation based on the JDBC metadata

returned by the JDBC driver.

hibernate.show_sql

Write all SQL statements to console. This is an alternative to setting the logcategory org.hibernate.SQL to debug.

e.g. true | false

hibernate.format_sql Pretty print the SQL in the log and console. e.g. true | false

hibernate.default_schemaQualify unqualified table names with the given schema/tablespace ingenerated SQL. e.g. SCHEMA_NAME

hibernate.default_catalogQualifies unqualified table names with the given catalog in generated SQL.e.g. CATALOG_NAME

hibernate.session_factory_name

The org.hibernate.SessionFactory will be automatically bound to this name in

JNDI after it has been created.

e.g. jndi/composite/name

hibernate.max_fetch_depth

Sets a maximum "depth" for the outer join fetch tree for single-endedassociations (one-to-one, many-to-one). A 0 disables default outer join

fetching.

e.g. recommended values between 0 and 3

hibernate.default_batch_fetch_sizeSets a default size for Hibernate batch fetching of associations. e.g.recommended values 4, 8, 16

hibernate.default_entity_mode

Sets a default mode for entity representation for all sessions opened from thisSessionFactory, defaults to pojo.

e.g. dynamic-map | pojo

hibernate.order_updatesForces Hibernate to order SQL updates by the primary key value of the itemsbeing updated. This will result in fewer transaction deadlocks in highlyconcurrent systems. e.g. true | false

hibernate.generate_statisticsIf enabled, Hibernate will collect statistics useful for performance tuning. e.g.true | false

hibernate.use_identifier_rollbackIf enabled, generated identifier properties will be reset to default values whenobjects are deleted. e.g. true | false

hibernate.use_sql_comments

If turned on, Hibernate will generate comments inside the SQL, for easierdebugging, defaults to false.

e.g. true | false

hibernate.id.new_generator_mappings

Setting is relevant when using @GeneratedValue. It indicates whether or not the

n e w IdentifierGenerator implementations are used for

javax.persistence.GenerationType.AUTO, javax.persistence.GenerationType.TABLE

a n d javax.persistence.GenerationType.SEQUENCE. Default to false to keep

backward compatibility.

e.g. true | false

Note

We recommend all new projects which make use of to use @GeneratedValue to also set

hibernate.id.new_generator_mappings=true as the new generators are more efficient and

closer to the JPA 2 specification semantic. However they are not backward compatiblewith existing databases (if a sequence or a table is used for id generation).

Table 3.4. Hibernate JDBC and Connection Properties


hibernate.jdbc.fetch_sizeA non-zero value determines the JDBC fetch size (callsStatement.setFetchSize()).

hibernate.jdbc.batch_sizeA non-zero value enables use of JDBC2 batch updates by Hibernate. e.g.recommended values between 5 and 30

hibernate.jdbc.batch_versioned_data

Set this property to true if your JDBC driver returns correct row counts from

executeBatch(). It is usually safe to turn this option on. Hibernate will then use

batched DML for automatically versioned data. Defaults to false.

e.g. true | false

hibernate.jdbc.factory_class

Select a custom org.hibernate.jdbc.Batcher. Most applications will not need this

configuration property.

e.g. classname.of.BatcherFactory

hibernate.jdbc.use_scrollable_resultsetEnables use of JDBC2 scrollable resultsets by Hibernate. This property isonly necessary when using user-supplied JDBC connections. Hibernateuses connection metadata otherwise. e.g. true | false

hibernate.jdbc.use_streams_for_binary

Use streams when writing/reading binary o r serializable types to/from JDBC.

*system-level property*

e.g. true | false

hibernate.jdbc.use_get_generated_keys

Enables use of JDBC3 PreparedStatement.getGeneratedKeys() to retrieve

natively generated keys after insert. Requires JDBC3+ driver and JRE1.4+,set to false if your driver has problems with the Hibernate identifiergenerators. By default, it tries to determine the driver capabilities usingconnection metadata.

e.g. true|false

hibernate.connection.provider_class

The classname of a custom org.hibernate.connection.ConnectionProvider which

provides JDBC connections to Hibernate.

e.g. classname.of.ConnectionProvider

hibernate.connection.isolation

Sets the JDBC transaction isolation level. Check java.sql.Connection for

meaningful values, but note that most databases do not support all isolationlevels and some define additional, non-standard isolations.

e.g. 1, 2, 4, 8

hibernate.connection.autocommitEnables autocommit for JDBC pooled connections (it is not recommended).e.g. true | false

hibernate.connection.release_mode

Specifies when Hibernate should release JDBC connections. By default, aJDBC connection is held until the session is explicitly closed ordisconnected. For an application server JTA datasource, use after_statement

to aggressively release connections after every JDBC call. For a non-JTAconnection, it often makes sense to release the connection at the end ofeach transaction, by using after_transaction. auto will choose after_statement for

the JTA and CMT transaction strategies and after_transaction for the JDBC

transaction strategy.

e.g. auto (default) | on_close | after_transaction | after_statement

This setting only affects Sessions returned from SessionFactory.openSession.

F o r Sessions obtained through SessionFactory.getCurrentSession, the

CurrentSessionContext implementation configured for use controls the

connection release mode for those Sessions. See Section 2.2, “Contextual

sessions”

hibernate.connection.<propertyName> Pass the JDBC property <propertyName> to DriverManager.getConnection().

hibernate.jndi.<propertyName> Pass the property <propertyName> to the JNDI InitialContextFactory.


Table 3.5. Hibernate Cache Properties


hibernate.cache.provider_class

The classname of a custom CacheProvider.

e.g. classname.of.CacheProvider

hibernate.cache.use_minimal_puts

Optimizes second-level cache operation to minimize writes, at thecost of more frequent reads. This setting is most useful for clusteredcaches and, in Hibernate, is enabled by default for clustered cacheimplementations. e.g. true|false

hibernate.cache.use_query_cacheEnables the query cache. Individual queries still have to be setcachable. e.g. true|false

hibernate.cache.use_second_level_cache

Can be used to completely disable the second level cache, which isenabled by default for classes which specify a <cache> mapping.

e.g. true|false

hibernate.cache.query_cache_factory

The classname of a custom QueryCache interface, defaults to the

built-in StandardQueryCache.

e.g. classname.of.QueryCache

hibernate.cache.region_prefix A prefix to use for second-level cache region names. e.g. prefix

hibernate.cache.use_structured_entriesForces Hibernate to store data in the second-level cache in a morehuman-friendly format. e.g. true|false

hibernate.cache.auto_evict_collection_cache

Enables the automatic eviction of a bi-directional association'scollection cache when an element in the ManyToOne collection isadded/updated/removed without properly managing the change onthe OneToMany side. e.g. true|false (default: false)

hibernate.cache.default_cache_concurrency_strategy

Setting used to give the name of the defaultorg.hibernate.annotations.CacheConcurrencyStrategy to use when either

@Cacheable or @Cache is used. @Cache(strategy="..") is used to

override this default.

Table 3.6. Hibernate Transaction Properties


hibernate.transaction.factory_class

The classname of a TransactionFactory to use with Hibernate Transaction API

(defaults to JDBCTransactionFactory).

e.g. classname.of.TransactionFactory

jta.UserTransaction

A JNDI name used by JTATransactionFactory to obtain the JTA

UserTransaction from the application server.

e.g. jndi/composite/name

hibernate.transaction.manager_lookup_class

The classname of a TransactionManagerLookup. It is required when JVM-level

caching is enabled or when using hilo generator in a JTA environment.

e.g. classname.of.TransactionManagerLookup

hibernate.transaction.flush_before_completion

If enabled, the session will be automatically flushed during the beforecompletion phase of the transaction. Built-in and automatic session contextmanagement is preferred, see Section 2.2, “Contextual sessions”.

e.g. true | false

hibernate.transaction.auto_close_session

If enabled, the session will be automatically closed during the aftercompletion phase of the transaction. Built-in and automatic session contextmanagement is preferred, see Section 2.2, “Contextual sessions”.

e.g. true | false

Table 3.7. Miscellaneous Properties


hibernate.current_session_context_class

Supply a custom strategy for the scoping of the "current" Session. See

Section 2.2, “Contextual sessions” for more information about the built-instrategies.

e.g. jta | thread | managed | custom.Class

hibernate.query.factory_class

Chooses the HQL parser implementation. e.g.org.hibernate.hql.internal.ast.ASTQueryTranslatorFactory or

org.hibernate.hql.internal.classic.ClassicQueryTranslatorFactory

hibernate.query.substitutions

Is used to map from tokens in Hibernate queries to SQL tokens (tokens mightbe function or literal names, for example). e.g.hqlLiteral=SQL_LITERAL, hqlFunction=SQLFUNC

hibernate.hbm2ddl.auto

Automatically validates or exports schema DDL to the database when theSessionFactory is created. Wi th create-drop, the database schema will be

dropped when the SessionFactory is closed explicitly.

e.g. validate | update | create | create-drop

hibernate.hbm2ddl.import_files

Comma-separated names of the optional files containing SQL DMLstatements executed during the SessionFactory creation. This is useful for

testing or demoing: by adding INSERT statements for example you canpopulate your database with a minimal set of data when it is deployed.

File order matters, the statements of a give file are executed before thestatements of the following files. These statements are only executed if theschema is created ie if hibernate.hbm2ddl.auto is set to create or create-drop.

e.g. /humans.sql,/dogs.sql

hibernate.hbm2ddl.import_files_sql_extractor

The classname of a custom ImportSqlCommandExtractor (defaults to the built-in

SingleLineSqlCommandExtractor) . This is useful for implementing dedicated

parser that extracts single SQL statements from each import file. Hibernatep r o v i d e s a l s o MultipleLinesSqlCommandExtractor which supports

instructions/comments and quoted strings spread over multiple lines(mandatory semicolon at the end of each statement).

e.g. classname.of.ImportSqlCommandExtractor

hibernate.bytecode.use_reflection_optimizer

Enables the use of bytecode manipulation instead of runtime reflection. Thisis a System-level property and cannot be set in hibernate.cfg.xml. Reflection

can sometimes be useful when troubleshooting. Hibernate always requiresjavassist even if you turn off the optimizer.

e.g. true | false

hibernate.bytecode.provider

At the moment, javassist is the only supported bytecode provider.

e.g. javassist

3.4.1. SQL Dialects

Always set the hibernate.dialect property to the correct org.hibernate.dialect.Dialect subclass for your database. If you specify a

dialect, Hibernate will use sensible defaults for some of the other properties listed above. This means that you will nothave to specify them manually.

Table 3.8. Hibernate SQL Dialects (hibernate.dialect)

RDBMS Dialect

CUBRID 8.3 and later org.hibernate.dialect.CUBRIDDialect

DB2 org.hibernate.dialect.DB2Dialect

DB2 AS/400 org.hibernate.dialect.DB2400Dialect

DB2 OS390 org.hibernate.dialect.DB2390Dialect

Firebird org.hibernate.dialect.FirebirdDialect

FrontBase org.hibernate.dialect.FrontbaseDialect

H2 org.hibernate.dialect.H2Dialect

HyperSQL (HSQL) org.hibernate.dialect.HSQLDialect

Informix org.hibernate.dialect.InformixDialect

Ingres org.hibernate.dialect.IngresDialect

Ingres 9 org.hibernate.dialect.Ingres9Dialect

Ingres 10 org.hibernate.dialect.Ingres10Dialect

Interbase org.hibernate.dialect.InterbaseDialect

InterSystems Cache 2007.1 org.hibernate.dialect.Cache71Dialect

JDataStore org.hibernate.dialect.JDataStoreDialect

Mckoi SQL org.hibernate.dialect.MckoiDialect

Microsoft SQL Server 2000 org.hibernate.dialect.SQLServerDialect

Microsoft SQL Server 2005 org.hibernate.dialect.SQLServer2005Dialect



Mimer SQL org.hibernate.dialect.MimerSQLDialect

MySQL org.hibernate.dialect.MySQLDialect

MySQL with InnoDB org.hibernate.dialect.MySQLInnoDBDialect

MySQL with MyISAM org.hibernate.dialect.MySQLMyISAMDialect

MySQL5 org.hibernate.dialect.MySQL5Dialect

MySQL5 with InnoDB org.hibernate.dialect.MySQL5InnoDBDialect

Oracle 8i org.hibernate.dialect.Oracle8iDialect

Oracle 9i org.hibernate.dialect.Oracle9iDialect

Oracle 10g and later org.hibernate.dialect.Oracle10gDialect

Oracle TimesTen org.hibernate.dialect.TimesTenDialect

Pointbase org.hibernate.dialect.PointbaseDialect

PostgreSQL 8.1 org.hibernate.dialect.PostgreSQL81Dialect

PostgreSQL 8.2 org.hibernate.dialect.PostgreSQL82Dialect

PostgreSQL 9 and later org.hibernate.dialect.PostgreSQL9Dialect

Progress org.hibernate.dialect.ProgressDialect

SAP DB org.hibernate.dialect.SAPDBDialect

SAP HANA (column store) org.hibernate.dialect.HANAColumnStoreDialect

SAP HANA (row store) org.hibernate.dialect.HANARowStoreDialect

Sybase org.hibernate.dialect.SybaseDialect

Sybase 11 org.hibernate.dialect.Sybase11Dialect

Sybase ASE 15.5 org.hibernate.dialect.SybaseASE15Dialect

Sybase ASE 15.7 org.hibernate.dialect.SybaseASE157Dialect

Sybase Anywhere org.hibernate.dialect.SybaseAnywhereDialect

Teradata org.hibernate.dialect.TeradataDialect

Unisys OS 2200 RDMS org.hibernate.dialect.RDMSOS2200Dialect

3.4.2. Outer Join Fetching

If your database supports ANSI, Oracle or Sybase style outer joins, outer join fetching will often increase performance bylimiting the number of round trips to and from the database. This is, however, at the cost of possibly more work performedby the database itself. Outer join fetching allows a whole graph of objects connected by many-to-one, one-to-many, many-to-many and one-to-one associations to be retrieved in a single SQL SELECT.

Outer join fetching can be disabled globally by setting the property hibernate.max_fetch_depth to 0. A setting of 1 or higher

enables outer join fetching for one-to-one and many-to-one associations that have been mapped with fetch="join".

See Section 20.1, “Fetching strategies” for more information.

3.4.3. Binary Streams

Oracle limits the size of byte arrays that can be passed to and/or from its JDBC driver. If you wish to use large instances of

binary or serializable type, you should enable hibernate.jdbc.use_streams_for_binary. This is a system-level setting only.

3.4.4. Second-level and query cache

The properties prefixed by hibernate.cache allow you to use a process or cluster scoped second-level cache system with

Hibernate. See the Section 20.2, “The Second Level Cache” for more information.

3.4.5. Query Language Substitution

You can define new Hibernate query tokens using hibernate.query.substitutions. For example:

hibernate.query.substitutions true=1, false=0

This would cause the tokens true and false to be translated to integer literals in the generated SQL.

hibernate.query.substitutions toLowercase=LOWER

This would allow you to rename the SQL LOWER function.

3.4.6. Hibernate statistics

If you enable hibernate.generate_statistics, Hibernate exposes a number of metrics that are useful when tuning a running

system via SessionFactory.getStatistics(). Hibernate can even be configured to expose these statistics via JMX. Read the

Javadoc of the interfaces in org.hibernate.stats for more information.

3.5. Logging

Important

Completely out of date. Hibernate uses JBoss Logging starting in 4.0. This will getdocumented as we migrate this content to the Developer Guide.

Hibernate utilizes Simple Logging Facade for Java (SLF4J) in order to log various system events. SLF4J can direct yourlogging output to several logging frameworks (NOP, Simple, log4j version 1.2, JDK 1.4 logging, JCL or logback)depending on your chosen binding. In order to setup logging you will need slf4j-api.jar in your classpath together with the

jar file for your preferred binding - slf4j-log4j12.jar in the case of Log4J. See the SLF4J documentation for more detail. To

use Log4j you will also need to place a log4j.properties file in your classpath. An example properties file is distributed with

Hibernate in the src/ directory.

It is recommended that you familiarize yourself with Hibernate's log messages. A lot of work has been put into making theHibernate log as detailed as possible, without making it unreadable. It is an essential troubleshooting device. The mostinteresting log categories are the following:

Table 3.9. Hibernate Log Categories

Category Function

org.hibernate.SQL Log all SQL DML statements as they are executed

org.hibernate.type Log all JDBC parameters

org.hibernate.tool.hbm2ddl Log all SQL DDL statements as they are executed

org.hibernate.pretty Log the state of all entities (max 20 entities) associated with the session at flush time

org.hibernate.cache Log all second-level cache activity

org.hibernate.transaction Log transaction related activity

org.hibernate.jdbc Log all JDBC resource acquisition

org.hibernate.hql.internal.ast.AST Log HQL and SQL ASTs during query parsing

org.hibernate.secure Log all JAAS authorization requests

org.hibernate Log everything. This is a lot of information but it is useful for troubleshooting

When developing applications with Hibernate, you should almost always work with debug enabled for the category

org.hibernate.SQL, or, alternatively, the property hibernate.show_sql enabled.

3.6. Implementing a NamingStrategy

The interface org.hibernate.cfg.NamingStrategy allows you to specify a "naming standard" for database objects and schema

elements.

You can provide rules for automatically generating database identifiers from Java identifiers or for processing "logical"column and table names given in the mapping file into "physical" table and column names. This feature helps reduce theverbosity of the mapping document, eliminating repetitive noise (TBL_ prefixes, for example). The default strategy used by

Hibernate is quite minimal.

You can specify a different strategy by calling Configuration.setNamingStrategy() before adding mappings:

SessionFactory sf = new Configuration() .setNamingStrategy(ImprovedNamingStrategy.INSTANCE) .addFile("Item.hbm.xml") .addFile("Bid.hbm.xml") .buildSessionFactory();

org.hibernate.cfg.ImprovedNamingStrategy is a built-in strategy that might be a useful starting point for some applications.

3.7. Implementing a PersisterClassProvider

You can configure the persister implementation used to persist your entities and collections:

by default, Hibernate uses persisters that make sense in a relational model and follow Java Persistence's specification

you can define a PersisterClassProvider implementation that provides the persister class used of a given entity or

http://www.slf4j.org/

http://www.slf4j.org/manual.html

collection

finally, you can override them on a per entity and collection basis in the mapping using @Persister or its XML

equivalent

The latter in the list the higher in priority.

You can pass the PersisterClassProvider instance to the Configuration object.

SessionFactory sf = new Configuration() .setPersisterClassProvider(customPersisterClassProvider) .addAnnotatedClass(Order.class) .buildSessionFactory();

The persister class provider methods, when returning a non null persister class, override the default Hibernate persisters.The entity name or the collection role are passed to the methods. It is a nice way to centralize the overriding logic of thepersisters instead of spreading them on each entity or collection mapping.

3.8. XML configuration file

An alternative approach to configuration is to specify a full configuration in a file named hibernate.cfg.xml. This file can be

used as a replacement for the hibernate.properties file or, if both are present, to override properties.

The XML configuration file is by default expected to be in the root of your CLASSPATH. Here is an example:

<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

 <session-factory name="java:hibernate/SessionFactory">

 <property name="connection.datasource">java:/comp/env/jdbc/MyDB</property> <property name="dialect">org.hibernate.dialect.MySQLDialect</property> <property name="show_sql">false</property> <property name="transaction.factory_class"> org.hibernate.transaction.JTATransactionFactory </property> <property name="jta.UserTransaction">java:comp/UserTransaction</property>

 <mapping resource="org/hibernate/auction/Item.hbm.xml"/> <mapping resource="org/hibernate/auction/Bid.hbm.xml"/>

 <class-cache class="org.hibernate.auction.Item" usage="read-write"/> <class-cache class="org.hibernate.auction.Bid" usage="read-only"/> <collection-cache collection="org.hibernate.auction.Item.bids" usage="read-write"/>

</session-factory>

</hibernate-configuration>

The advantage of this approach is the externalization of the mapping file names to configuration. The hibernate.cfg.xml is

also more convenient once you have to tune the Hibernate cache. It is your choice to use either hibernate.properties or

hibernate.cfg.xml. Both are equivalent, except for the above mentioned benefits of using the XML syntax.

With the XML configuration, starting Hibernate is then as simple as:

SessionFactory sf = new Configuration().configure().buildSessionFactory();

You can select a different XML configuration file using:

SessionFactory sf = new Configuration() .configure("catdb.cfg.xml") .buildSessionFactory();

3.9. Java EE Application Server integration

Hibernate has the following integration points for J2EE infrastructure:

Container-managed datasources: Hibernate can use JDBC connections managed by the container and providedthrough JNDI. Usually, a JTA compatible TransactionManager and a ResourceManager take care of transaction

management (CMT), especially distributed transaction handling across several datasources. You can also demarcatetransaction boundaries programmatically (BMT), or you might want to use the optional Hibernate Transaction API for

this to keep your code portable.

Automatic JNDI binding: Hibernate can bind its SessionFactory to JNDI after startup.

JTA Session binding: the Hibernate Session can be automatically bound to the scope of JTA transactions. Simply

lookup the SessionFactory from JNDI and get the current Session. Let Hibernate manage flushing and closing the

Session when your JTA transaction completes. Transaction demarcation is either declarative (CMT) or programmatic

(BMT/UserTransaction).

JMX deployment: if you have a JMX capable application server (e.g. JBoss AS), you can choose to deploy Hibernateas a managed MBean. This saves you the one line startup code to build your SessionFactory from a Configuration. The

container will startup your HibernateService and also take care of service dependencies (datasource has to be available

before Hibernate starts, etc).

Depending on your environment, you might have to set the configuration option hibernate.connection.aggressive_release to

true if your application server shows "connection containment" exceptions.

3.9.1. Transaction strategy configuration

The Hibernate Session API is independent of any transaction demarcation system in your architecture. If you let Hibernate

use JDBC directly through a connection pool, you can begin and end your transactions by calling the JDBC API. If you runin a J2EE application server, you might want to use bean-managed transactions and call the JTA API and UserTransaction

when needed.

To keep your code portable between these two (and other) environments we recommend the optional HibernateTransaction API, which wraps and hides the underlying system. You have to specify a factory class for Transaction instances

by setting the Hibernate configuration property hibernate.transaction.factory_class.

There are three standard, or built-in, choices:

org.hibernate.transaction.JDBCTransactionFactory

delegates to database (JDBC) transactions (default)

org.hibernate.transaction.JTATransactionFactory

delegates to container-managed transactions if an existing transaction is underway in this context (for example, EJBsession bean method). Otherwise, a new transaction is started and bean-managed transactions are used.

org.hibernate.transaction.CMTTransactionFactory

delegates to container-managed JTA transactions

You can also define your own transaction strategies (for a CORBA transaction service, for example).

Some features in Hibernate (i.e., the second level cache, Contextual Sessions with JTA, etc.) require access to the JTATransactionManager in a managed environment. In an application server, since J2EE does not standardize a single

mechanism, you have to specify how Hibernate should obtain a reference to the TransactionManager:

Table 3.10. JTA TransactionManagers

Transaction Factory Application Server

org.hibernate.transaction.JBossTransactionManagerLookup JBoss AS

org.hibernate.transaction.WeblogicTransactionManagerLookup Weblogic

org.hibernate.transaction.WebSphereTransactionManagerLookup WebSphere

org.hibernate.transaction.WebSphereExtendedJTATransactionLookup WebSphere 6

org.hibernate.transaction.OrionTransactionManagerLookup Orion

org.hibernate.transaction.ResinTransactionManagerLookup Resin

org.hibernate.transaction.JOTMTransactionManagerLookup JOTM

org.hibernate.transaction.JOnASTransactionManagerLookup JOnAS

org.hibernate.transaction.JRun4TransactionManagerLookup JRun4

org.hibernate.transaction.BESTransactionManagerLookup Borland ES

org.hibernate.transaction.JBossTSStandaloneTransactionManagerLookup

JBoss TS used standalone (ie. outside JBoss ASand a JNDI environment generally). Known to

work for org.jboss.jbossts:jbossjta:4.11.0.Final

3.9.2. JNDI-bound SessionFactory

A JNDI-bound Hibernate SessionFactory can simplify the lookup function of the factory and create new Sessions. This is not,

however, related to a JNDI bound Datasource; both simply use the same registry.

If you wish to have the SessionFactory bound to a JNDI namespace, specify a name (e.g. java:hibernate/SessionFactory)

using the property hibernate.session_factory_name. If this property is omitted, the SessionFactory will not be bound to JNDI.

This is especially useful in environments with a read-only JNDI default implementation (in Tomcat, for example).

When binding the SessionFactory to JNDI, Hibernate will use the values of hibernate.jndi.url, hibernate.jndi.class to instantiate

an initial context. If they are not specified, the default InitialContext will be used.

Hibernate will automatically place the SessionFactory in JNDI after you call cfg.buildSessionFactory(). This means you will

have this call in some startup code, or utility class in your application, unless you use JMX deployment with theHibernateService (this is discussed later in greater detail).

If you use a JNDI SessionFactory, an EJB or any other class, you can obtain the SessionFactory using a JNDI lookup.

It is recommended that you bind the SessionFactory to JNDI in a managed environment and use a static singleton

otherwise. To shield your application code from these details, we also recommend to hide the actual lookup code for aSessionFactory in a helper class, such as HibernateUtil.getSessionFactory(). Note that such a class is also a convenient way to

startup Hibernate—see chapter 1.


The easiest way to handle Sessions and transactions is Hibernate's automatic "current" Session management. For a

discussion of contextual sessions see Section 2.2, “Contextual sessions”. Using the "jta" session context, if there is no

Hibernate Session associated with the current JTA transaction, one will be started and associated with that JTA transaction

the first time you call sessionFactory.getCurrentSession(). The Sessions retrieved via getCurrentSession() in the "jta" context are

set to automatically flush before the transaction completes, close after the transaction completes, and aggressively releaseJDBC connections after each statement. This allows the Sessions to be managed by the life cycle of the JTA transaction to

which it is associated, keeping user code clean of such management concerns. Your code can either use JTAprogrammatically through UserTransaction, or (recommended for portable code) use the Hibernate Transaction API to set

transaction boundaries. If you run in an EJB container, declarative transaction demarcation with CMT is preferred.

Chapter 4. Persistent Classes

Table of Contents


4.1.1. Implement a no-argument constructor4.1.2. Provide an identifier property4.1.3. Prefer non-final classes (semi-optional)4.1.4. Declare accessors and mutators for persistent fields (optional)

4.2. Implementing inheritance4.3. Implementing equals() and hashCode()

4.4. Dynamic models4.5. Tuplizers4.6. EntityNameResolvers

Persistent classes are classes in an application that implement the entities of the business problem (e.g. Customer andOrder in an E-commerce application). The term "persistent" here means that the classes are able to be persisted, not thatthey are in the persistent state (see Section 11.1, “Hibernate object states” for discussion).

Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object (POJO)programming model. However, none of these rules are hard requirements. Indeed, Hibernate assumes very little about thenature of your persistent objects. You can express a domain model in other ways (using trees of java.util.Map instances, for

example).


Example 4.1. Simple POJO representing a cat

package eg;import java.util.Set;import java.util.Date;

public class Cat {private Long id; // identifier

private Date birthdate;private Color color;private char sex;private float weight; private int litterId;

private Cat mother; private Set kittens = new HashSet();

private void setId(Long id) { this.xml:id=id; } public Long getId() { return id; }

void setBirthdate(Date date) { birthdate = date; } public Date getBirthdate() { return birthdate; }

void setWeight(float weight) { this.weight = weight; } public float getWeight() { return weight; }

public Color getColor() { return color; } void setColor(Color color) { this.color = color; }

void setSex(char sex) { this.sex=sex; } public char getSex() { return sex; }

void setLitterId(int id) { this.litterId = id; } public int getLitterId() { return litterId; }

void setMother(Cat mother) { this.mother = mother; } public Cat getMother() { return mother; } void setKittens(Set kittens) { this.kittens = kittens; } public Set getKittens() { return kittens; }

// addKitten not needed by Hibernate public void addKitten(Cat kitten) { kitten.setMother(this); kitten.setLitterId( kittens.size() ); kittens.add(kitten); }}

The four main rules of persistent classes are explored in more detail in the following sections.

4.1.1. Implement a no-argument constructor

Cat has a no-argument constructor. All persistent classes must have a default constructor (which can be non-public) so

that Hibernate can instantiate them using java.lang.reflect.Constructor.newInstance(). It is recommended that this constructor

be defined with at least package visibility in order for runtime proxy generation to work properly.

4.1.2. Provide an identifier property

Note

Historically this was considered option. While still not (yet) enforced, this should beconsidered a deprecated feature as it will be completely required to provide a identifierproperty in an upcoming release.

Cat has a property named id. This property maps to the primary key column(s) of the underlying database table. The type of

the identifier property can be any "basic" type (see ???) . See Section 9.4, “Components as composite identifiers” forinformation on mapping composite (multi-column) identifiers.

Note

Identifiers do not necessarily need to identify column(s) in the database physicallydefined as a primary key. They should just identify columns that can be used to uniquelyidentify rows in the underlying table.

We recommend that you declare consistently-named identifier properties on persistent classes and that you use a nullable(i.e., non-primitive) type.

4.1.3. Prefer non-final classes (semi-optional)

A central feature of Hibernate, proxies (lazy loading), depends upon the persistent class being either non-final, or theimplementation of an interface that declares all public methods. You can persist final classes that do not implement an

interface with Hibernate; you will not, however, be able to use proxies for lazy association fetching which will ultimatelylimit your options for performance tuning. To persist a final class which does not implement a "full" interface you must

disable proxy generation. Se e Example 4.2, “Disabling proxies in hbm.xml” and Example 4.3, “Disabling proxies in

annotations”.

Example 4.2. Disabling proxies in hbm.xml

<class name="Cat" lazy="false"...>...</class>

Example 4.3. Disabling proxies in annotations

@Entity @Proxy(lazy=false) public class Cat { ... }

If the final class does implement a proper interface, you could alternatively tell Hibernate to use the interface instead when

generating the proxies. See Example 4.4, “Proxying an interface in hbm.xml” and Example 4.5, “Proxying an interface in

annotations”.

Example 4.4. Proxying an interface in hbm.xml

<class name="Cat" proxy="ICat"...>...</class>

Example 4.5. Proxying an interface in annotations

@Entity @Proxy(proxyClass=ICat.class) public class Cat implements ICat { ... }

You should also avoid declaring public final methods as this will again limit the ability to generate proxies from this class. If

you want to use a class with public final methods, you must explicitly disable proxying. Again, see Example 4.2, “Disabling

proxies in hbm.xml” and Example 4.3, “Disabling proxies in annotations”.

4.1.4. Declare accessors and mutators for persistent fields (optional)

Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance variables. It is

better to provide an indirection between the relational schema and internal data structures of the class. By default,Hibernate persists JavaBeans style properties and recognizes method names of the form getFoo, isFoo a n d setFoo. If

required, you can switch to direct field access for particular properties.

Properties need not be declared public. Hibernate can persist a property declared with package, protected or private visibility

as well.

4.2. Implementing inheritance

A subclass must also observe the first and second rules. It inherits its identifier property from the superclass, Cat. For

example:

package eg;

public class DomesticCat extends Cat { private String name;

public String getName() { return name; } protected void setName(String name) { this.name=name; }}

4.3. Implementing equals() and hashCode()

You have to override the equals() and hashCode() methods if you:

intend to put instances of persistent classes in a Set (the recommended way to represent many-valued associations);

and

intend to use reattachment of detached instances

Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a particular sessionscope. When you mix instances retrieved in different sessions, you must implement equals() and hashCode() if you wish to

have meaningful semantics for Sets.

The most obvious way is to implement equals()/hashCode() by comparing the identifier value of both objects. If the value is

the same, both must be the same database row, because they are equal. If both are added to a Set, you will only have one

element in the Set). Unfortunately, you cannot use that approach with generated identifiers. Hibernate will only assign

identifier values to objects that are persistent; a newly created instance will not have any identifier value. Furthermore, ifan instance is unsaved and currently in a Set, saving it will assign an identifier value to the object. If equals() and

hashCode() are based on the identifier value, the hash code would change, breaking the contract of the Set. See the

Hibernate website for a full discussion of this problem. This is not a Hibernate issue, but normal Java semantics of objectidentity and equality.

It is recommended that you implement equals() and hashCode() using Business key equality. Business key equality means

that the equals() method compares only the properties that form the business key. It is a key that would identify our instance

in the real world (a natural candidate key):

public class Cat {

... public boolean equals(Object other) { if (this == other) return true; if ( !(other instanceof Cat) ) return false;

final Cat cat = (Cat) other;

if ( !cat.getLitterId().equals( getLitterId() ) ) return false; if ( !cat.getMother().equals( getMother() ) ) return false;

return true; }

public int hashCode() { int result; result = getMother().hashCode(); result = 29 * result + getLitterId(); return result; }

}

A business key does not have to be as solid as a database primary key candidate (see Section 13.1.3, “Consideringobject identity”). Immutable or unique properties are usually good candidates for a business key.

4.4. Dynamic models

Note

The following features are currently considered experimental and may change in thenear future.

Persistent entities do not necessarily have to be represented as POJO classes or as JavaBean objects at runtime.Hibernate also supports dynamic models (using Maps of Maps at runtime). With this approach, you do not write persistent

classes, only mapping files.

By default, Hibernate works in normal POJO mode. You can set a default entity representation mode for a particularSessionFactory using the default_entity_mode configuration option (see Table 3.3, “Hibernate Configuration Properties”).

The following examples demonstrate the representation using Maps. First, in the mapping file an entity-name has to be

declared instead of, or in addition to, a class name:

<hibernate-mapping>

<class entity-name="Customer">

<id name="id" type="long" column="ID"> <generator class="sequence"/> </id>

<property name="name" column="NAME" type="string"/>

<property name="address" column="ADDRESS" type="string"/>

<many-to-one name="organization" column="ORGANIZATION_ID" class="Organization"/>

<bag name="orders" inverse="true" lazy="false" cascade="all"> <key column="CUSTOMER_ID"/> <one-to-many class="Order"/> </bag>

</class> </hibernate-mapping>

Even though associations are declared using target class names, the target type of associations can also be a dynamicentity instead of a POJO.

After setting the default entity mode to dynamic-map for the SessionFactory, you can, at runtime, work with Maps of Maps:

Session s = openSession();Transaction tx = s.beginTransaction();

// Create a customerMap david = new HashMap();david.put("name", "David");

// Create an organizationMap foobar = new HashMap();foobar.put("name", "Foobar Inc.");

// Link bothdavid.put("organization", foobar);

// Save boths.save("Customer", david);s.save("Organization", foobar);

tx.commit();s.close();

One of the main advantages of dynamic mapping is quick turnaround time for prototyping, without the need for entity classimplementation. However, you lose compile-time type checking and will likely deal with many exceptions at runtime. As aresult of the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a properdomain model implementation on top later on.

Entity representation modes can also be set on a per Session basis:

Session dynamicSession = pojoSession.getSession(EntityMode.MAP);

// Create a customerMap david = new HashMap();david.put("name", "David");dynamicSession.save("Customer", david);...dynamicSession.flush();dynamicSession.close()...// Continue on pojoSession

Please note that the call to getSession() using an EntityMode is on the Session API, not the SessionFactory. That way, the new

Session shares the underlying JDBC connection, transaction, and other context information. This means you do not have to

call flush() and close() on the secondary Session, and also leave the transaction and connection handling to the primary unit

of work.

4.5. Tuplizers

org.hibernate.tuple.Tuplizer and its sub-interfaces are responsible for managing a particular representation of a piece of data

given that representation's org.hibernate.EntityMode. If a given piece of data is thought of as a data structure, then a tuplizer

is the thing that knows how to create such a data structure, how to extract values from such a data structure and how toinject values into such a data structure. For example, for the POJO entity mode, the corresponding tuplizer knows howcreate the POJO through its constructor. It also knows how to access the POJO properties using the defined propertyaccessors.

There are two (high-level) types of Tuplizers:

org.hibernate.tuple.entity.EntityTuplizer which is responsible for managing the above mentioned contracts in regards to

entities

org.hibernate.tuple.component.ComponentTuplizer which does the same for components

Users can also plug in their own tuplizers. Perhaps you require that java.util.Map implementation other than java.util.HashMap

be used while in the dynamic-map entity-mode. Or perhaps you need to define a different proxy generation strategy thanthe one used by default. Both would be achieved by defining a custom tuplizer implementation. Tuplizer definitions areattached to the entity or component mapping they are meant to manage. Going back to the example of our Customer entity,

Example 4.6, “Specify custom tuplizers in annotations” shows how to specify a custom org.hibernate.tuple.entity.EntityTuplizer

using annotations while Example 4.7, “Specify custom tuplizers in hbm.xml” shows how to do the same in hbm.xml

Example 4.6. Specify custom tuplizers in annotations

@Entity@Tuplizer(impl = DynamicEntityTuplizer.class)public interface Cuisine { @Id @GeneratedValue public Long getId(); public void setId(Long id);

public String getName(); public void setName(String name);

@Tuplizer(impl = DynamicComponentTuplizer.class) public Country getCountry(); public void setCountry(Country country);}

Example 4.7. Specify custom tuplizers in hbm.xml

<hibernate-mapping> <class entity-name="Customer">  <tuplizer entity-mode="dynamic-map" class="CustomMapTuplizerImpl"/>

<id name="id" type="long" column="ID"> <generator class="sequence"/> </id>

 ... </class></hibernate-mapping>

4.6. EntityNameResolvers

org.hibernate.EntityNameResolver is a contract for resolving the entity name of a given entity instance. The interface defines a

single method resolveEntityName which is passed the entity instance and is expected to return the appropriate entity name

(null is allowed and would indicate that the resolver does not know how to resolve the entity name of the given entityinstance). Generally speaking, an org.hibernate.EntityNameResolver is going to be most useful in the case of dynamic

models. One example might be using proxied interfaces as your domain model. The hibernate test suite has an exampleof this exact style of usage under the org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from thatpackage for illustration.

/** * A very trivial JDK Proxy InvocationHandler implementation where we proxy an * interface as the domain model and simply store persistent state in an internal * Map. This is an extremely trivial example meant only for illustration. */public final class DataProxyHandler implements InvocationHandler { private String entityName; private HashMap data = new HashMap();

public DataProxyHandler(String entityName, Serializable id) { this.entityName = entityName; data.put( "Id", id ); }

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { String methodName = method.getName(); if ( methodName.startsWith( "set" ) ) { String propertyName = methodName.substring( 3 ); data.put( propertyName, args[0] ); } else if ( methodName.startsWith( "get" ) ) { String propertyName = methodName.substring( 3 ); return data.get( propertyName ); } else if ( "toString".equals( methodName ) ) { return entityName + "#" + data.get( "Id" ); } else if ( "hashCode".equals( methodName ) ) { return new Integer( this.hashCode() ); } return null; }

public String getEntityName() { return entityName; }

public HashMap getData() { return data; }}

public class ProxyHelper { public static String extractEntityName(Object object) { // Our custom java.lang.reflect.Proxy instances actually bundle // their appropriate entity name, so we simply extract it from there // if this represents one of our proxies; otherwise, we return null if ( Proxy.isProxyClass( object.getClass() ) ) { InvocationHandler handler = Proxy.getInvocationHandler( object ); if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) { DataProxyHandler myHandler = ( DataProxyHandler ) handler; return myHandler.getEntityName(); } } return null; }

// various other utility methods ....

}

/** * The EntityNameResolver implementation. * * IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names * should be resolved. Since this particular impl can handle resolution for all of our * entities we want to take advantage of the fact that SessionFactoryImpl keeps these * in a Set so that we only ever have one instance registered. Why? Well, when it * comes time to resolve an entity name, Hibernate must iterate over all the registered * resolvers. So keeping that number down helps that process be as speedy as possible. * Hence the equals and hashCode implementations as is */public class MyEntityNameResolver implements EntityNameResolver { public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();

public String resolveEntityName(Object entity) { return ProxyHelper.extractEntityName( entity ); }

public boolean equals(Object obj) { return getClass().equals( obj.getClass() ); }

public int hashCode() { return getClass().hashCode(); }}

public class MyEntityTuplizer extends PojoEntityTuplizer { public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) { super( entityMetamodel, mappedEntity ); }

public EntityNameResolver[] getEntityNameResolvers() { return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE }; }

public String determineConcreteSubclassEntityName(Object entityInstance, SessionFactoryImplementor factory) { String entityName = ProxyHelper.extractEntityName( entityInstance ); if ( entityName == null ) { entityName = super.determineConcreteSubclassEntityName( entityInstance, factory ); } return entityName; }

...

In order to register an org.hibernate.EntityNameResolver users must either:

1. Implement a custom tuplizer (see Section 4.5, “Tuplizers”), implementing the getEntityNameResolvers method

2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the implementation class for

org.hibernate.SessionFactory) using the registerEntityNameResolver method.

Chapter 5. Basic O/R Mapping

Table of Contents


5.1.1. Entity5.1.2. Identifiers5.1.3. Optimistic locking properties (optional)5.1.4. Property5.1.5. Embedded objects (aka components)5.1.6. Inheritance strategy5.1.7. Mapping one to one and one to many associations5.1.8. Natural-id5.1.9. Any5.1.10. Properties5.1.11. Some hbm.xml specificities


5.2.1. Entities and values5.2.2. Basic value types5.2.3. Custom value types

5.3. Mapping a class more than once5.4. SQL quoted identifiers5.5. Generated properties5.6. Column transformers: read and write expressions5.7. Auxiliary database objects


Object/relational mappings can be defined in three approaches:

using Java 5 annotations (via the Java Persistence 2 annotations)

using JPA 2 XML deployment descriptors (described in chapter XXX)

using the Hibernate legacy XML files approach known as hbm.xml

Annotations are split in two categories, the logical mapping annotations (describing the object model, the associationbetween two entities etc.) and the physical mapping annotations (describing the physical schema, tables, columns,indexes, etc). We will mix annotations from both categories in the following code examples.

JPA annotations are in the javax.persistence.* package. Hibernate specific extensions are in org.hibernate.annotations.*. You

favorite IDE can auto-complete annotations and their attributes for you (even without a specific "JPA" plugin, since JPAannotations are plain Java 5 annotations).

Here is an example of mapping

package eg;

@Entity @Table(name="cats") @Inheritance(strategy=SINGLE_TABLE)@DiscriminatorValue("C") @DiscriminatorColumn(name="subclass", discriminatorType=CHAR)public class Cat { @Id @GeneratedValue public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } private Integer id;

public BigDecimal getWeight() { return weight; } public void setWeight(BigDecimal weight) { this.weight = weight; } private BigDecimal weight;

@Temporal(DATE) @NotNull @Column(updatable=false) public Date getBirthdate() { return birthdate; } public void setBirthdate(Date birthdate) { this.birthdate = birthdate; } private Date birthdate;

@org.hibernate.annotations.Type(type="eg.types.ColorUserType") @NotNull @Column(updatable=false) public ColorType getColor() { return color; } public void setColor(ColorType color) { this.color = color; } private ColorType color;

@NotNull @Column(updatable=false) public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } private String sex;

@NotNull @Column(updatable=false) public Integer getLitterId() { return litterId; } public void setLitterId(Integer litterId) { this.litterId = litterId; } private Integer litterId;

@ManyToOne @JoinColumn(name="mother_id", updatable=false) public Cat getMother() { return mother; } public void setMother(Cat mother) { this.mother = mother; } private Cat mother;

@OneToMany(mappedBy="mother") @OrderBy("litterId") public Set<Cat> getKittens() { return kittens; } public void setKittens(Set<Cat> kittens) { this.kittens = kittens; } private Set<Cat> kittens = new HashSet<Cat>();}

@Entity @DiscriminatorValue("D")public class DomesticCat extends Cat {

public String getName() { return name; } public void setName(String name) { this.name = name } private String name;}

@Entitypublic class Dog { ... }

The legacy hbm.xml approach uses an XML schema designed to be readable and hand-editable. The mapping languageis Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.

Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generatethe mapping document. These include XDoclet, Middlegen and AndroMDA.

Here is an example mapping:


<hibernate-mapping package="eg">

<class name="Cat" table="cats" discriminator-value="C">

<id name="id"> <generator class="native"/> </id>

<discriminator column="subclass" type="character"/>

<property name="weight"/>

<property name="birthdate" type="date" not-null="true" update="false"/>

<property name="color" type="eg.types.ColorUserType" not-null="true" update="false"/>

<property name="sex" not-null="true" update="false"/>

<property name="litterId" column="litterId" update="false"/>

<many-to-one name="mother" column="mother_id" update="false"/>

<set name="kittens" inverse="true" order-by="litter_id"> <key column="mother_id"/> <one-to-many class="Cat"/> </set>

<subclass name="DomesticCat" discriminator-value="D">

<property name="name" type="string"/>

</subclass>

</class>

<class name="Dog">  </class>


We will now discuss the concepts of the mapping documents (both annotations and XML). We will only describe, however,the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains someextra optional attributes and elements that affect the database schemas exported by the schema export tool (for example,the not-null attribute).

5.1.1. Entity

An entity is a regular Java object (aka POJO) which will be persisted by Hibernate.

To mark an object as an entity in annotations, use the @Entity annotation.

@Entitypublic class Flight implements Serializable { Long id;

@Id public Long getId() { return id; }

public void setId(Long id) { this.id = id; }}

That's pretty much it, the rest is optional. There are however any options to tweak your entity mapping, let's explore them.

@Table lets you define the table the entity will be persisted into. If undefined, the table name is the unqualified class name

of the entity. You can also optionally define the catalog, the schema as well as unique constraints on the table.

@Entity@Table(name="TBL_FLIGHT", schema="AIR_COMMAND", uniqueConstraints= @UniqueConstraint( name="flight_number", columnNames={"comp_prefix", "flight_number"} ) )public class Flight implements Serializable { @Column(name="comp_prefix") public String getCompagnyPrefix() { return companyPrefix; }

@Column(name="flight_number") public String getNumber() { return number; }}

The constraint name is optional (generated if left undefined). The column names composing the constraint correspond tothe column names as defined before the Hibernate NamingStrategy is applied.

Tip

Be sure to use the database-level column names for the columnNames property of a

@UniqueConstraint. For example, whilst for simple types the database-level column name

may be the same as the entity-level property name, this is often not the case for relationalproperties.

@Entity.name lets you define the shortcut name of the entity you can used in JP-QL and HQL queries. It defaults to the

unqualified class name of the class.

Hibernate goes beyond the JPA specification and provide additional configurations. Some of them are hosted [email protected]:

dynamicInsert / dynamicUpdate (defaults to false): specifies that INSERT / UPDATE SQL should be generated at runtime

and contain only the columns whose values are not null. The dynamic-update and dynamic-insert settings are not

inherited by subclasses. Although these settings can increase performance in some cases, they can actually decreaseperformance in others.

selectBeforeUpdate (defaults to false): specifies that Hibernate should never perform an SQL UPDATE unless it is certain

that an object is actually modified. Only when a transient object has been associated with a new session usingupdate(), will Hibernate perform an extra SQL SELECT to determine if an UPDATE is actually required. Use of

select-before-update will usually decrease performance. It is useful to prevent a database update trigger being called

unnecessarily if you reattach a graph of detached instances to a Session.

polymorphisms (defaults to IMPLICIT): determines whether implicit or explicit query polymorphisms is used. Implicit

polymorphisms means that instances of the class will be returned by a query that names any superclass orimplemented interface or class, and that instances of any subclass of the class will be returned by a query that namesthe class itself. Explicit polymorphisms means that class instances will be returned only by queries that explicitly namethat class. Queries that name the class will return only instances of subclasses mapped. For most purposes, thedefault polymorphisms=IMPLICIT is appropriate. Explicit polymorphisms is useful when two different classes are mapped

to the same table This allows a "lightweight" class that contains a subset of the table columns.

persister: specifies a custom ClassPersister. The persister attribute lets you customize the persistence strategy used for

the class. You can, for example, specify your own subclass of org.hibernate.persister.EntityPersister, or you can even

provide a completely new implementation of the interface org.hibernate.persister.ClassPersister that implements, for

example, persistence via stored procedure calls, serialization to flat files or LDAP. See org.hibernate.test.CustomPersister

for a simple example of "persistence" to a Hashtable.

optimisticLock (defaults to VERSION): determines the optimistic locking strategy. If you enable dynamicUpdate, you will

have a choice of optimistic locking strategies:

version: check the version/timestamp columns

all: check all columns

dirty: check the changed columns, allowing some concurrent updates

none: do not use optimistic locking

It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategyoptimizes performance and correctly handles modifications made to detached instances (i.e. when Session.merge() is

used).

Tip

Be sure to import @javax.persistence.Entity to mark a class as an entity. It's a common

mistake to import @org.hibernate.annotations.Entity by accident.

Some entities are not mutable. They cannot be updated by the application. This allows Hibernate to make some minorperformance optimizations.. Use the @Immutable annotation.

You can also alter how Hibernate deals with lazy initialization for this class. On @Proxy, use lazy=false to disable lazy

fetching (not recommended). You can also specify an interface to use for lazy initializing proxies (defaults to the classitself): use proxyClass on @Proxy. Hibernate will initially return proxies ( using bytecode provider defined by

hibernate.bytecode.provider) that implement the named interface. The persistent object will load when a method of the proxy

is invoked. See "Initializing collections and proxies" below.

@BatchSize specifies a "batch size" for fetching instances of this class by identifier. Not yet loaded instances are loaded

batch-size at a time (default 1).

You can specific an arbitrary SQL WHERE condition to be used when retrieving objects of this class. Use @Where for that.

In the same vein, @Check lets you define an SQL expression used to generate a multi-row check constraint for automatic

schema generation.

There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level,although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but youcannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-onlyentity to a given SQL subselect expression using @org.hibernate.annotations.Subselect:

@Entity@Subselect("select item.name, max(bid.amount), count(*) " + "from item " + "join bid on bid.item_id = item.id " + "group by item.name")@Synchronize( {"item", "bid"} ) //tables impactedpublic class Summary { @Id public String getId() { return id; } ...}

Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against thederived entity do not return stale data. The <subselect> is available both as an attribute and a nested mapping element.

We will now explore the same options using the hbm.xml structure. You can declare a persistent class using the class

element. For example:

<class name="ClassName" table="tableName" discriminator-value="discriminator_value" mutable="true|false" schema="owner" catalog="catalog" proxy="ProxyInterface" dynamic-update="true|false" dynamic-insert="true|false" select-before-update="true|false" polymorphism="implicit|explicit" where="arbitrary sql where condition" persister="PersisterClass" batch-size="N" optimistic-lock="none|version|dirty|all" lazy="true|false" entity-name="EntityName" check="arbitrary sql check condition" rowxml:id="rowid" subselect="SQL expression" abstract="true|false" node="element-name"/>

name (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is missing, it

is assumed that the mapping is for a non-POJO entity.

table (optional - defaults to the unqualified class name): the name of its database table.

discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses that is

used for polymorphic behavior. Acceptable values include null and not null.

mutable (optional - defaults to true): specifies that instances of the class are (not) mutable.

schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.

catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.

proxy (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of the class

itself.

dynamic-update (optional - defaults to false): specifies that UPDATE SQL should be generated at runtime and can

contain only those columns whose values have changed.

dynamic-insert (optional - defaults to false): specifies that INSERT SQL should be generated at runtime and contain

only the columns whose values are not null.

select-before-update (optional - defaults to false): specifies that Hibernate should never perform an SQL UPDATE

unless it is certain that an object is actually modified. Only when a transient object has been associated with anew session using update(), will Hibernate perform an extra SQL SELECT to determine if an UPDATE is actually

required.

polymorphisms (optional - defaults to implicit): determines whether implicit or explicit query polymorphisms is used.

where (optional): specifies an arbitrary SQL WHERE condition to be used when retrieving objects of this class.

persister (optional): specifies a custom ClassPersister.

batch-size (optional - defaults to 1): specifies a "batch size" for fetching instances of this class by identifier.

optimistic-lock (optional - defaults to version): determines the optimistic locking strategy.

(16) lazy (optional): lazy fetching can be disabled by setting lazy="false".

(17) entity-name (optional - defaults to the class name): Hibernate allows a class to be mapped multiple times,

potentially to different tables. It also allows entity mappings that are represented by Maps or XML at the Javalevel. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamicmodels” and ??? for more information.

(18) check (optional): an SQL expression used to generate a multi-row check constraint for automatic schema

generation.

(19) rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use the rowid

extra column for fast updates once this option has been set to rowid. A ROWID is an implementation detail and

represents the physical location of a stored tuple.

(20) subselect (optional): maps an immutable and read-only entity to a database subselect. This is useful if you want to

have a view instead of a base table. See below for more information.

(21) abstract (optional): is used to mark abstract superclasses in <union-subclass> hierarchies.

It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interfaceusing the <subclass> element. You can persist any static inner class. Specify the class name using the standard form i.e.

e.g.Foo$Bar.

Here is how to do a virtual view (subselect) in XML:

<class name="Summary"> <subselect> select item.name, max(bid.amount), count(*) from item join bid on bid.item_id = item.id group by item.name </subselect> <synchronize table="item"/> <synchronize table="bid"/> <id name="name"/> ...</class>

The <subselect> is available both as an attribute and a nested mapping element.

5.1.2. Identifiers

Mapped classes must declare the primary key column of the database table. Most classes will also have a JavaBeans-style property holding the unique identifier of an instance.

Mark the identifier property with @Id.

(16)(17)(18)(19)(20)(21)

@Entitypublic class Person { @Id Integer getId() { ... } ...}

In hbm.xml, use the <id> element which defines the mapping from that property to the primary key column.

<id name="propertyName" type="typename" column="column_name" unsaved-value="null|any|none|undefined|id_value" access="field|property|ClassName"> node="element-name|@attribute-name|element/@attribute|."

<generator class="generatorClass"/></id>

name (optional): the name of the identifier property.

type (optional): a name that indicates the Hibernate type.

column (optional - defaults to the property name): the name of the primary key column.

unsaved-value (optional - defaults to a "sensible" value): an identifier property value that indicates an instance is

newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previoussession.

access (optional - defaults to property): the strategy Hibernate should use for accessing the property value.

If the name attribute is missing, it is assumed that the class has no identifier property.

The unsaved-value attribute is almost never needed in Hibernate and indeed has no corresponding element in annotations.

You can also declare the identifier as a composite identifier. This allows access to legacy data with composite keys. Its useis strongly discouraged for anything else.

5.1.2.1. Composite identifier

You can define a composite primary key through several syntaxes:

use a component type to represent the identifier and map it as a property in the entity: you then annotated the propertyas @EmbeddedId. The component type has to be Serializable.

map multiple properties as @Id properties: the identifier type is then the entity class itself and needs to be Serializable.

This approach is unfortunately not standard and only supported by Hibernate.

map multiple properties as @Id properties and declare an external class to be the identifier type. This class, which

needs to be Serializable, is declared on the entity via the @IdClass annotation. The identifier type must contain the same

properties as the identifier properties of the entity: each property name must be the same, its type must be the same aswell if the entity property is of a basic type, its type must be the type of the primary key of the associated entity if theentity property is an association (either a @OneToOne or a @ManyToOne).

As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backwardcompatibilities and we recommend you not to use it (for simplicity sake).

Let's explore all three cases using examples.

5.1.2.1.1. id as a property using a component type

Here is a simple example of @EmbeddedId.

@Entityclass User { @EmbeddedId @AttributeOverride(name="firstName", column=@Column(name="fld_firstname") UserId id;

Integer age;}

@Embeddableclass UserId implements Serializable { String firstName; String lastName;}

You can notice that the UserId class is serializable. To override the column mapping, use @AttributeOverride.

An embedded id can itself contains the primary key of an associated entity.

@Entityclass Customer { @EmbeddedId CustomerId id; boolean preferredCustomer;

@MapsId("userId") @JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") }) @OneToOne User user;}

@Embeddableclass CustomerId implements Serializable { UserId userId; String customerNumber;

//implements equals and hashCode}

@Entity class User { @EmbeddedId UserId id; Integer age;}

@Embeddableclass UserId implements Serializable { String firstName; String lastName;


In the embedded id object, the association is represented as the identifier of the associated entity. But you can link itsvalue to a regular association in the entity via the @MapsId annotation. The @MapsId value correspond to the property

n a me of the embedded id object containing the associated entity's identifier. In the database, it means that theCustomer.user and the CustomerId.userId properties share the same underlying column (user_fk in this case).

Tip

The component type used as identifier must implement equals() and hashCode().

In practice, your code only sets the Customer.user property and the user id value is copied by Hibernate into the

CustomerId.userId property.

Warning

The id value can be copied as late as flush time, don't rely on it until after flush time.

While not supported in JPA, Hibernate lets you place your association directly in the embedded id component (instead ofhaving to use the @MapsId annotation).

@Entityclass Customer { @EmbeddedId CustomerId id; boolean preferredCustomer;}

@Embeddableclass CustomerId implements Serializable { @OneToOne @JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") }) User user; String customerNumber;





Let's now rewrite these examples using the hbm.xml syntax.

<composite-id name="propertyName" class="ClassName" mapped="true|false" access="field|property|ClassName" node="element-name|.">

<key-property name="propertyName" type="typename" column="column_name"/> <key-many-to-one name="propertyName" class="ClassName" column="column_name"/> ......</composite-id>

First a simple example:

<class name="User"> <composite-id name="id" class="UserId"> <key-property name="firstName" column="fld_firstname"/> <key-property name="lastName"/> </composite-id></class>

Then an example showing how an association can be mapped.

<class name="Customer"> <composite-id name="id" class="CustomerId"> <key-property name="firstName" column="userfirstname_fk"/> <key-property name="lastName" column="userfirstname_fk"/> <key-property name="customerNumber"/> </composite-id>

<property name="preferredCustomer"/>

<many-to-one name="user"> <column name="userfirstname_fk" updatable="false" insertable="false"/> <column name="userlastname_fk" updatable="false" insertable="false"/> </many-to-one></class>

<class name="User"> <composite-id name="id" class="UserId"> <key-property name="firstName"/> <key-property name="lastName"/> </composite-id>

<property name="age"/></class>

Notice a few things in the previous example:

the order of the properties (and column) matters. It must be the same between the association and the primary key ofthe associated entity

the many to one uses the same columns as the primary key and thus must be marked as read only (insertable and

updatable to false).

unlike with @MapsId, the id value of the associated entity is not transparently copied, check the foreign id generator for

more information.

The last example shows how to map association directly in the embedded id component.

<class name="Customer"> <composite-id name="id" class="CustomerId"> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id>

<property name="preferredCustomer"/></class>



This is the recommended approach to map composite identifier. The following options should not be considered unlesssome constraint are present.

5.1.2.1.2. Multiple id properties without identifier type

Another, arguably more natural, approach is to place @Id on multiple properties of your entity. This approach is only

supported by Hibernate (not JPA compliant) but does not require an extra embeddable component.

@Entityclass Customer implements Serializable { @Id @OneToOne @JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") }) User user; @Id String customerNumber;

boolean preferredCustomer;





In this case Customer is its own identifier representation: it must implement Serializable and must implement equals() and

hashCode().

In hbm.xml, the same mapping is:

<class name="Customer"> <composite-id> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id>




5.1.2.1.3. Multiple id properties with with a dedicated identifier type

@IdClass on an entity points to the class (component) representing the identifier of the class. The properties marked @Id

on the entity must have their corresponding property on the @IdClass. The return type of search twin property must be

either identical for basic properties or must correspond to the identifier class of the associated entity for an association.

Warning

This approach is inherited from the EJB 2 days and we recommend against its use. But,after all it's your application and Hibernate supports it.

@Entity@IdClass(CustomerId.class)class Customer implements Serializable { @Id @OneToOne @JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") }) User user; @Id String customerNumber;

boolean preferredCustomer;}

class CustomerId implements Serializable { UserId user; String customerNumber;


@Entity class User { @EmbeddedId UserId id; Integer age;




Customer and CustomerId do have the same properties customerNumber as well as user. CustomerId must be Serializable and

implement equals() and hashCode().

While not JPA standard, Hibernate let's you declare the vanilla associated property in the @IdClass.

@Entity@IdClass(CustomerId.class)class Customer implements Serializable { @Id @OneToOne @JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") }) User user; @Id String customerNumber;

boolean preferredCustomer;}

class CustomerId implements Serializable { @OneToOne User user; String customerNumber;


@Entity class User { @EmbeddedId UserId id; Integer age;


@Embeddableclass UserId implements Serializable { String firstName; String lastName;}

This feature is of limited interest though as you are likely to have chosen the @IdClass approach to stay JPA compliant or

you have a quite twisted mind.

Here are the equivalent on hbm.xml files:

<class name="Customer"> <composite-id class="CustomerId" mapped="true"> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id>




5.1.2.2. Identifier generator

Hibernate can generate and populate identifier values for you automatically. This is the recommended approach over"business" or "natural" id (especially composite ids).

Hibernate offers various generation strategies, let's explore the most common ones first that happens to be standardizedby JPA:

IDENTITY: supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returnedidentifier is of type long, short or int.

SEQUENCE (called seqhilo in Hibernate): uses a hi/lo algorithm to efficiently generate identifiers of type long, short or

int, given a named database sequence.

TABLE (called MultipleHiLoPerTableGenerator in Hibernate) : uses a hi/lo algorithm to efficiently generate identifiers of

type long, short or int, given a table and column as a source of hi values. The hi/lo algorithm generates identifiers that

are unique only for a particular database.

AUTO: selects IDENTITY, SEQUENCE or TABLE depending upon the capabilities of the underlying database.

Important

We recommend all new projects to use the new enhanced identifier generators. They aredeactivated by default for entities using annotations but can be activated usinghibernate.id.new_generator_mappings=true. These new generators are more efficient and

closer to the JPA 2 specification semantic.However they are not backward compatible with existing Hibernate based application (ifa sequence or a table is used for id generation). See XXXXXXX ??? for more informationon how to activate them.

To mark an id property as generated, use the @GeneratedValue annotation. You can specify the strategy used (default to

AUTO) by setting strategy.

@Entitypublic class Customer { @Id @GeneratedValue Integer getId() { ... };}

@Entity public class Invoice { @Id @GeneratedValue(strategy=GenerationType.IDENTITY) Integer getId() { ... };}

SEQUENCE and TABLE require additional configurations that you can set using @SequenceGenerator and @TableGenerator:

name: name of the generator

table / sequenceName: name of the table or the sequence (defaulting respectively to hibernate_sequences and

hibernate_sequence)

catalog / schema:

initialValue: the value from which the id is to start generating

allocationSize: the amount to increment by when allocating id numbers from the generator

In addition, the TABLE strategy also let you customize:

pkColumnName: the column name containing the entity identifier

valueColumnName: the column name containing the identifier value

pkColumnValue: the entity identifier

uniqueConstraints: any potential column constraint on the table containing the ids

To link a table or sequence generator definition with an actual generated property, use the same name in both thedefinition name and the generator value generator as shown below.

@Id @GeneratedValue( strategy=GenerationType.SEQUENCE, generator="SEQ_GEN")@javax.persistence.SequenceGenerator( name="SEQ_GEN", sequenceName="my_sequence", allocationSize=20)public Integer getId() { ... }

The scope of a generator definition can be the application or the class. Class-defined generators are not visible outsidethe class and can override application level generators. Application level generators are defined in JPA's XMLdeployment descriptors (see XXXXXX ???):

<table-generator name="EMP_GEN" table="GENERATOR_TABLE" pk-column-name="key" value-column-name="hi" pk-column-value="EMP" allocation-size="20"/>

//and the annotation equivalent

@javax.persistence.TableGenerator( name="EMP_GEN", table="GENERATOR_TABLE", pkColumnName = "key", valueColumnName = "hi" pkColumnValue="EMP", allocationSize=20)

<sequence-generator name="SEQ_GEN" sequence-name="my_sequence" allocation-size="20"/>

//and the annotation equivalent

@javax.persistence.SequenceGenerator( name="SEQ_GEN", sequenceName="my_sequence", allocationSize=20)

If a JPA XML descriptor (like META-INF/orm.xml) is used to define the generators, EMP_GEN and SEQ_GEN are application

level generators.

Note

Package level definition is not supported by the JPA specification. However, you can usethe @GenericGenerator at the package level (see ???).

These are the four standard JPA generators. Hibernate goes beyond that and provide additional generators or additionaloptions as we will see below. You can also write your own custom identifier generator by implementingorg.hibernate.id.IdentifierGenerator.

To define a custom generator, use the @GenericGenerator annotation (and its plural counter part @GenericGenerators) that

describes the class of the identifier generator or its short cut name (as described below) and a list of key/value parameters.When using @GenericGenerator and assigning it via @GeneratedValue.generator, the @GeneratedValue.strategy is ignored:

leave it blank.

@Id @GeneratedValue(generator="system-uuid")@GenericGenerator(name="system-uuid", strategy = "uuid")public String getId() {

@Id @GeneratedValue(generator="trigger-generated")@GenericGenerator( name="trigger-generated", strategy = "select", parameters = @Parameter(name="key", value = "socialSecurityNumber"))public String getId() {

The hbm.xml approach uses the optional <generator> child element inside <id>. If any parameters are required to configure

or initialize the generator instance, they are passed using the <param> element.

<id name="id" type="long" column="cat_id"> <generator class="org.hibernate.id.TableHiLoGenerator"> <param name="table">uid_table</param> <param name="column">next_hi_value_column</param> </generator></id>

5.1.2.2.1. Various additional generators

All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface. Some applications

can choose to provide their own specialized implementations, however, Hibernate provides a range of built-inimplementations. The shortcut names for the built-in generators are as follows:

increment

generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same

table. Do not use in a cluster.

identity

supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is oftype long, short or int.

sequence

uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is oftype long, short or int

hilo

uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default

hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that

are unique only for a particular database.

seqhilo

uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.

uuid

Generates a 128-bit UUID based on a custom algorithm. The value generated is represented as a string of 32hexidecimal digits. Users can also configure it to use a separator (config parameter "separator") which separates thehexidecimal digits into 8{sep}8{sep}4{sep}8{sep}4. Note specifically that this is different than the IETF RFC 4122representation of 8-4-4-4-12. If you need RFC 4122 compliant UUIDs, consider using "uuid2" generator discussedbelow.

uuid2

Generates a IETF RFC 4122 compliant (variant 2) 128-bit UUID. The exact "version" (the RFC term) generateddepends on the pluggable "generation strategy" used (see below). Capable of generating values as java.util.UUID,

java.lang.String or as a byte array of length 16 (byte[16]). The "generation strategy" is defined by the interface

org.hibernate.id.UUIDGenerationStrategy. The generator defines 2 configuration parameters for defining which generation

strategy to use:

uuid_gen_strategy_class

Names the UUIDGenerationStrategy class to use

uuid_gen_strategy

Names the UUIDGenerationStrategy instance to use

Out of the box, comes with the following strategies:

org.hibernate.id.uuid.StandardRandomStrategy (the default) - generates "version 3" (aka, "random") UUID values via

the randomUUID method of java.util.UUID

org.hibernate.id.uuid.CustomVersionOneStrategy - generates "version 1" UUID values, using IP address since mac

address not available. If you need mac address to be used, consider leveraging one of the existing third partyUUID generators which sniff out mac address and integrating it via the org.hibernate.id.UUIDGenerationStrategy

contract. Two such libraries known at time of this writing include http://johannburkard.de/software/uuid/ andhttp://commons.apache.org/sandbox/id/uuid.html

guid

uses a database-generated GUID string on MS SQL Server and MySQL.

native

selects identity, sequence or hilo depending upon the capabilities of the underlying database.

assigned

lets the application assign an identifier to the object before save() is called. This is the default strategy if no <generator>

element is specified.

select

retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving theprimary key value.

foreign

uses the identifier of another associated object. It is usually used in conjunction with a <one-to-one> primary key

association.

sequence-identity

a specialized sequence generation strategy that utilizes a database sequence for the actual value generation, butcombines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statementexecution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insertstatements are disabled due to a bug in the Oracle drivers.

5.1.2.2.2. Hi/lo algorithm

The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm. The first implementation

requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.

<id name="id" type="long" column="cat_id"> <generator class="hilo"> <param name="table">hi_value</param> <param name="column">next_value</param> <param name="max_lo">100</param> </generator></id>

<id name="id" type="long" column="cat_id"> <generator class="seqhilo"> <param name="sequence">hi_value</param> <param name="max_lo">100</param> </generator></id>

Unfortunately, you cannot use hilo when supplying your own Connection to Hibernate. When Hibernate uses an application

server datasource to obtain connections enlisted with JTA, you must configure thehibernate.transaction.manager_lookup_class.

5.1.2.2.3. UUID algorithm

http://johannburkard.de/software/uuid/

http://commons.apache.org/sandbox/id/uuid.html

The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a countervalue that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so thisis the best option without using JNI.

5.1.2.2.4. Identity columns and sequences

For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use identity key generation. For

databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use sequence style key

generation. Both of these strategies require two SQL queries to insert a new object. For example:

<id name="id" type="long" column="person_id"> <generator class="sequence"> <param name="sequence">person_id_sequence</param> </generator></id>

<id name="id" type="long" column="person_id" unsaved-value="0"> <generator class="identity"/></id>

For cross-platform development, the native strategy will, depending on the capabilities of the underlying database, choose

from the identity, sequence and hilo strategies.

5.1.2.2.5. Assigned identifiers

If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use the assigned

generator. This special generator uses the identifier value already assigned to the object's identifier property. Thegenerator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you donot specify @GeneratedValue nor <generator> elements.

The assigned generator makes Hibernate use unsaved-value="undefined". This forces Hibernate to go to the database to

determine if an instance is transient or detached, unless there is a version or timestamp property, or you defineInterceptor.isUnsaved().

5.1.2.2.6. Primary keys assigned by triggers

Hibernate does not generate DDL with triggers. It is for legacy schemas only.

<id name="id" type="long" column="person_id"> <generator class="select"> <param name="key">socialSecurityNumber</param> </generator></id>

In the above example, there is a unique valued property named socialSecurityNumber. It is defined by the class, as a natural

key and a surrogate key named person_id, whose value is generated by a trigger.

5.1.2.2.7. Identity copy (foreign generator)

Finally, you can ask Hibernate to copy the identifier from another associated entity. In the Hibernate jargon, it is known asa foreign generator but the JPA mapping reads better and is encouraged.

@Entityclass MedicalHistory implements Serializable { @Id @OneToOne @JoinColumn(name = "person_id") Person patient;}

@Entitypublic class Person implements Serializable { @Id @GeneratedValue Integer id;}

Or alternatively

@Entityclass MedicalHistory implements Serializable { @Id Integer id;

@MapsId @OneToOne @JoinColumn(name = "patient_id") Person patient;}

@Entityclass Person { @Id @GeneratedValue Integer id;}

In hbm.xml use the following approach:

<class name="MedicalHistory"> <id name="id"> <generator class="foreign"> <param name="property">patient</param> </generator> </id> <one-to-one name="patient" class="Person" constrained="true"/></class>

5.1.2.3. Enhanced identifier generators

Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifiergeneration. The first aspect is database portability; the second is optimization Optimization means that you do not have toquery the database for every request for a new identifier value. These two new generators are intended to take the placeof some of the named generators described above, starting in 3.3.x. However, they are included in the current releasesand can be referenced by FQN.

The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator which is intended, firstly, as a

replacement for the sequence generator and, secondly, as a better portability generator than native. This is because native

generally chooses between identity and sequence which have largely different semantics that can cause subtle issues in

applications eyeing portability. org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a different

manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on thecapabilities of the dialect being used. The difference between this and native is that table-based and sequence-based

storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-basedgenerators. This generator has a number of configuration parameters:

sequence_name (optional, defaults to hibernate_sequence): the name of the sequence or table to be used.

initial_value (optional, defaults to 1): the initial value to be retrieved from the sequence/table. In sequence creation

terms, this is analogous to the clause typically named "STARTS WITH".

increment_size (optional - defaults to 1): the value by which subsequent calls to the sequence/table should differ. In

sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".

force_table_use (optional - defaults to false): should we force the use of a table as the backing structure even though the

dialect might support sequence?

value_column (optional - defaults to next_val): only relevant for table structures, it is the name of the column on the table

which is used to hold the value.

prefer_sequence_per_entity (optional - defaults to false): should we create separate sequence for each entity that share

current generator based on its name?

sequence_per_entity_suffix (optional - defaults to _SEQ): suffix added to the name of a dedicated sequence.

optimizer (optional - defaults to none): See Section 5.1.2.3.1, “Identifier generator optimization”

The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is intended, firstly, as a replacement

for the table generator, even though it actually functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator, and

secondly, as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable

optimizers. Essentially this generator defines a table capable of holding a number of different increment valuessimultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:

table_name (optional - defaults to hibernate_sequences): the name of the table to be used.

value_column_name (optional - defaults to next_val): the name of the column on the table that is used to hold the value.

segment_column_name (optional - defaults to sequence_name): the name of the column on the table that is used to hold

the "segment key". This is the value which identifies which increment value to use.

segment_value (optional - defaults to default): The "segment key" value for the segment from which we want to pull

increment values for this generator.

segment_value_length (optional - defaults to 255): Used for schema generation; the column size to create this segment

key column.

initial_value (optional - defaults to 1): The initial value to be retrieved from the table.

increment_size (optional - defaults to 1): The value by which subsequent calls to the table should differ.

optimizer (optional - defaults to ??): See Section 5.1.2.3.1, “Identifier generator optimization”.

5.1.2.3.1. Identifier generator optimization

For identifier generators that store values in the database, it is inefficient for them to hit the database on each and everycall to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database whenyou have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the twoenhanced generators (Section 5.1.2.3, “Enhanced identifier generators” support this operation.

none (generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the

database for each and every request.

hilo: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are

expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "groupnumber". The increment_size is multiplied by that value in memory to define a group "hi value".

pooled: as with the case of hilo, this optimizer attempts to minimize the number of hits to the database. Here, however,

we simply store the starting value for the "next group" into the database structure rather than a sequential value incombination with an in-memory grouping algorithm. Here, increment_size refers to the values coming from the

database.

5.1.2.4. Partial identifier generation

Hibernate supports the automatic generation of some of the identifier properties. Simply use the @GeneratedValue

annotation on one or several id properties.

Warning

The Hibernate team has always felt such a construct as fundamentally wrong. Try hard tofix your data model before using this feature.

@Entitypublic class CustomerInventory implements Serializable { @Id @TableGenerator(name = "inventory", table = "U_SEQUENCES", pkColumnName = "S_ID", valueColumnName = "S_NEXTNUM", pkColumnValue = "inventory", allocationSize = 1000) @GeneratedValue(strategy = GenerationType.TABLE, generator = "inventory") Integer id;

@Id @ManyToOne(cascade = CascadeType.MERGE) Customer customer;}

@Entitypublic class Customer implements Serializable { @Id private int id;}

You can also generate properties inside an @EmbeddedId class.

5.1.3. Optimistic locking properties (optional)

When using long transactions or conversations that span several database transactions, it is useful to store versioningdata to ensure that if the same entity is updated by two conversations, the last to commit changes will be informed and notoverride the other conversation's work. It guarantees some isolation while still allowing for good scalability and worksparticularly well in read-often write-sometimes situations.

You can use two approaches: a dedicated version number or a timestamp.

A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with anull version or timestamp as transient, irrespective of what other unsaved-value strategies are specified. Declaring a

nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It isespecially useful for people using assigned identifiers or composite keys.

5.1.3.1. Version number

You can add optimistic locking capability to an entity using the @Version annotation:

@Entitypublic class Flight implements Serializable {... @Version @Column(name="OPTLOCK") public Integer getVersion() { ... }}

The version property will be mapped to the OPTLOCK column, and the entity manager will use it to detect conflicting

updates (preventing lost updates you might otherwise see with the last-commit-wins strategy).

The version column may be a numeric. Hibernate supports any kind of type provided that you define and implement theappropriate UserVersionType.

The application must not alter the version number set up by Hibernate in any way. To artificially increase the versionnumber, check in Hibernate Entity Manager's reference documentation LockModeType.OPTIMISTIC_FORCE_INCREMENT or

LockModeType.PESSIMISTIC_FORCE_INCREMENT.

If the version number is generated by the database (via a trigger for example), make sure to [email protected](GenerationTime.ALWAYS).

To declare a version property in hbm.xml, use:

<version column="version_column" name="propertyName" type="typename" access="field|property|ClassName" unsaved-value="null|negative|undefined" generated="never|always" insert="true|false" node="element-name|@attribute-name|element/@attribute|."/>

column (optional - defaults to the property name): the name of the column holding the version number.

name: the name of a property of the persistent class.

type (optional - defaults to integer): the type of the version number.

access (optional - defaults to property): the strategy Hibernate uses to access the property value.

unsaved-value (optional - defaults to undefined): a version property value that indicates that an instance is newly

instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.Undefined specifies that the identifier property value should be used.

generated (optional - defaults to never): specifies that this version property value is generated by the database. See

the discussion of generated properties for more information.

insert (optional - defaults to true): specifies whether the version column should be included in SQL insert

statements. It can be set to false if the database column is defined with a default value of 0.

5.1.3.2. Timestamp

Alternatively, you can use a timestamp. Timestamps are a less safe implementation of optimistic locking. However,sometimes an application might use the timestamps in other ways as well.

Simply mark a property of type Date or Calendar as @Version.

@Entitypublic class Flight implements Serializable {... @Version public Date getLastUpdate() { ... }}

When using timestamp versioning you can tell Hibernate where to retrieve the timestamp value from - database or JVM -by optionally adding the @org.hibernate.annotations.Source annotation to the property. Possible values for the value attribute

o f the annotation are org.hibernate.annotations.SourceType.VM and org.hibernate.annotations.SourceType.DB. The default is

SourceType.DB which is also used in case there is no @Source annotation at all.

Like in the case of version numbers, the timestamp can also be generated by the database instead of Hibernate. To dothat, use @org.hibernate.annotations.Generated(GenerationTime.ALWAYS).

In hbm.xml, use the <timestamp> element:

<timestamp column="timestamp_column" name="propertyName" access="field|property|ClassName" unsaved-value="null|undefined" source="vm|db" generated="never|always" node="element-name|@attribute-name|element/@attribute|."/>

column (optional - defaults to the property name): the name of a column holding the timestamp.

name: the name of a JavaBeans style property of Java type Date or Timestamp of the persistent class.

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

unsaved-value (optional - defaults to null): a version property value that indicates that an instance is newly

instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.Undefined specifies that the identifier property value should be used.

source (optional - defaults to vm): Where should Hibernate retrieve the timestamp value from? From the database,

or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit thedatabase in order to determine the "next value". It is safer to use in clustered environments. Not all Dialects are

known to support the retrieval of the database's current timestamp. Others may also be unsafe for usage in lockingdue to lack of precision (Oracle 8, for example).

generated (optional - defaults to never): specifies that this timestamp property value is actually generated by the

database. See the discussion of generated properties for more information.

Note

<Timestamp> is equivalent to <version type="timestamp">. And <timestamp source="db"> is

equivalent to <version type="dbtimestamp">

5.1.4. Property

You need to decide which property needs to be made persistent in a given entity. This differs slightly between theannotation driven metadata and the hbm.xml files.

5.1.4.1. Property mapping with annotations

In the annotations world, every non static non transient property (field or method depending on the access type) of anentity is considered persistent, unless you annotate it as @Transient. Not having an annotation for your property is

equivalent to the appropriate @Basic annotation.

The @Basic annotation allows you to declare the fetching strategy for a property. If set to LAZY, specifies that this property

should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation, ifyour classes are not instrumented, property level lazy loading is silently ignored. The default is EAGER. You can also mark

a property as not optional thanks to the @Basic.optional attribute. This will ensure that the underlying column are not

nullable (if possible). Note that a better approach is to use the @NotNull annotation of the Bean Validation specification.

Let's look at a few examples:

public transient int counter; //transient property

private String firstname; //persistent property

@TransientString getLengthInMeter() { ... } //transient property

String getName() {... } // persistent property

@Basicint getLength() { ... } // persistent property

@Basic(fetch = FetchType.LAZY)String getDetailedComment() { ... } // persistent property

@Temporal(TemporalType.TIME)java.util.Date getDepartureTime() { ... } // persistent property

@Enumerated(EnumType.STRING)Starred getNote() { ... } //enum persisted as String in database

counter, a transient field, and lengthInMeter, a method annotated as @Transient, and will be ignored by the Hibernate. name,

length, and firstname properties are mapped persistent and eagerly fetched (the default for simple properties). The

detailedComment property value will be lazily fetched from the database once a lazy property of the entity is accessed for the

first time. Usually you don't need to lazy simple properties (not to be confused with lazy association fetching). Therecommended alternative is to use the projection capability of JP-QL (Java Persistence Query Language) or Criteria

queries.

JPA support property mapping of all basic types supported by Hibernate (all basic Java types , their respective wrappersand serializable classes). Hibernate Annotations supports out of the box enum type mapping either into a ordinal column(saving the enum ordinal) or a string based column (saving the enum string representation): the persistencerepresentation, defaulted to ordinal, can be overridden through the @Enumerated annotation as shown in the note property

example.

In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal data you might want todescribe the expected precision in database. Temporal data can have DATE, TIME, or TIMESTAMP precision (ie the actual

date, only the time, or both). Use the @Temporal annotation to fine tune that.

@Lob indicates that the property should be persisted in a Blob or a Clob depending on the property type: java.sql.Clob,

Character[], char[] and java.lang.String will be persisted in a Clob. java.sql.Blob, Byte[], byte[] and Serializable type will be

persisted in a Blob.

@Lobpublic String getFullText() { return fullText;}

@Lobpublic byte[] getFullCode() { return fullCode;}

If the property type implements java.io.Serializable and is not a basic type, and if the property is not annotated with @Lob,

then the Hibernate serializable type is used.

5.1.4.1.1. Type

You can also manually specify a type using the @org.hibernate.annotations.Type and some parameters if needed.

@Type.type could be:

1. The name of a Hibernate basic type: integer, string, character, date, timestamp, float, binary, serializable, object, blob etc.

2. The name of a Java class with a default basic type: int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.

3. The name of a serializable Java class.

4. The class name of a custom type: com.illflow.type.MyCustomType etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type.Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4.

@org.hibernate.annotations.TypeDef and @org.hibernate.annotations.TypeDefs allows you to declare type definitions. These

annotations can be placed at the class or package level. Note that these definitions are global for the session factory(even when defined at the class level). If the type is used on a single entity, you can place the definition on the entity itself.Otherwise, it is recommended to place the definition at the package level. In the example below, when Hibernateencounters a property of class PhoneNumer, it delegates the persistence strategy to the custom mapping type

PhoneNumberType. However, properties belonging to other classes, too, can delegate their persistence strategy to

PhoneNumberType, by explicitly using the @Type annotation.

Note

Package level annotations are placed in a file named package-info.java in the

appropriate package. Place your annotations before the package declaration.

@TypeDef( name = "phoneNumber", defaultForType = PhoneNumber.class, typeClass = PhoneNumberType.class)

@Entitypublic class ContactDetails { [...] private PhoneNumber localPhoneNumber; @Type(type="phoneNumber") private OverseasPhoneNumber overseasPhoneNumber; [...]}

The following example shows the usage of the parameters attribute to customize the TypeDef.

//in org/hibernate/test/annotations/entity/package-info.java@TypeDefs( { @TypeDef( name="caster", typeClass = CasterStringType.class, parameters = { @Parameter(name="cast", value="lower") } ) })package org.hibernate.test.annotations.entity;

//in org/hibernate/test/annotations/entity/Forest.javapublic class Forest { @Type(type="caster") public String getSmallText() { ...}

When using composite user type, you will have to express column definitions. The @Columns has been introduced for that

purpose.

@Type(type="org.hibernate.test.annotations.entity.MonetaryAmountUserType")@Columns(columns = { @Column(name="r_amount"), @Column(name="r_currency")})public MonetaryAmount getAmount() { return amount;}

public class MonetaryAmount implements Serializable { private BigDecimal amount; private Currency currency; ...}

5.1.4.1.2. Access type

By default the access type of a class hierarchy is defined by the position of the @Id or @EmbeddedId annotations. If these

annotations are on a field, then only fields are considered for persistence and the state is accessed via the field. If theseannotations are on a getter, then only the getters are considered for persistence and the state is accessed via thegetter/setter. That works well in practice and is the recommended approach.

Note

The placement of annotations within a class hierarchy has to be consistent (either field oron property) to be able to determine the default access type. It is recommended to stick toone single annotation placement strategy throughout your whole application.

However in some situations, you need to:

force the access type of the entity hierarchy

override the access type of a specific entity in the class hierarchy

override the access type of an embeddable type

The best use case is an embeddable class used by several entities that might not use the same access type. In this case itis better to force the access type at the embeddable class level.

To force the access type on a given class, use the @Access annotation as showed below:

@Entitypublic class Order { @Id private Long id; public Long getId() { return id; } public void setId(Long id) { this.id = id; }

@Embedded private Address address; public Address getAddress() { return address; } public void setAddress() { this.address = address; }}

@Entitypublic class User { private Long id; @Id public Long getId() { return id; } public void setId(Long id) { this.id = id; }

private Address address; @Embedded public Address getAddress() { return address; } public void setAddress() { this.address = address; }}

@Embeddable@Access(AcessType.PROPERTY)public class Address { private String street1; public String getStreet1() { return street1; } public void setStreet1() { this.street1 = street1; }

private hashCode; //not persistent}

You can also override the access type of a single property while keeping the other properties standard.

@Entitypublic class Order { @Id private Long id; public Long getId() { return id; } public void setId(Long id) { this.id = id; } @Transient private String userId; @Transient private String orderId;

@Access(AccessType.PROPERTY) public String getOrderNumber() { return userId + ":" + orderId; } public void setOrderNumber() { this.userId = ...; this.orderId = ...; }}

In this example, the default access type is FIELD except for the orderNumber property. Note that the corresponding field, if

any must be marked as @Transient or transient.

@org.hibernate.annotations.AccessType

The annotation @org.hibernate.annotations.AccessType should be considered deprecated

for FIELD and PROPERTY access. It is still useful however if you need to use a customaccess type.

5.1.4.1.3. Optimistic lock

It is sometimes useful to avoid increasing the version number even if a given property is dirty (particularly collections). Youcan do that by annotating the property (or collection) with @OptimisticLock(excluded=true).

More formally, specifies that updates to this property do not require acquisition of the optimistic lock.

5.1.4.1.4. Declaring column attributes

The column(s) used for a property mapping can be defined using the @Column annotation. Use it to override default values

(see the JPA specification for more information on the defaults). You can use this annotation at the property level forproperties that are:

not annotated at all

annotated with @Basic

annotated with @Version

annotated with @Lob

annotated with @Temporal

@Entitypublic class Flight implements Serializable {...@Column(updatable = false, name = "flight_name", nullable = false, length=50)public String getName() { ... }

The name property is mapped to the flight_name column, which is not nullable, has a length of 50 and is not updatable

(making the property immutable).

This annotation can be applied to regular properties as well as @Id or @Version properties.

@Column( name="columnName"; boolean unique() default false; boolean nullable() default true; boolean insertable() default true; boolean updatable() default true; String columnDefinition() default ""; String table() default ""; int length() default 255; int precision() default 0; // decimal precision int scale() default 0; // decimal scale

name (optional): the column name (default to the property name)

unique (optional): set a unique constraint on this column or not (default false)

nullable (optional): set the column as nullable (default true).

insertable (optional): whether or not the column will be part of the insert statement (default true)

updatable (optional): whether or not the column will be part of the update statement (default true)

columnDefinition (optional): override the sql DDL fragment for this particular column (non portable)

table (optional): define the targeted table (default primary table)

length (optional): column length (default 255)

precision (optional): column decimal precision (default 0)

scale (optional): column decimal scale if useful (default 0)

5.1.4.1.5. Formula

Sometimes, you want the Database to do some computation for you rather than in the JVM, you might also create somekind of virtual column. You can use a SQL fragment (aka formula) instead of mapping a property into a column. This kindof property is read only (its value is calculated by your formula fragment).

@Formula("obj_length * obj_height * obj_width")public long getObjectVolume()

The SQL fragment can be as complex as you want and even include subselects.

5.1.4.1.6. Non-annotated property defaults

If a property is not annotated, the following rules apply:

If the property is of a single type, it is mapped as @Basic

Otherwise, if the type of the property is annotated as @Embeddable, it is mapped as @Embedded

Otherwise, if the type of the property is Serializable, it is mapped as @Basic in a column holding the object in its

serialized version

Otherwise, if the type of the property is java.sql.Clob or java.sql.Blob, it is mapped as @Lob with the appropriate LobType

5.1.4.2. Property mapping with hbm.xml

The <property> element declares a persistent JavaBean style property of the class.

<property name="propertyName" column="column_name" type="typename" update="true|false" insert="true|false" formula="arbitrary SQL expression" access="field|property|ClassName" lazy="true|false" unique="true|false" not-null="true|false" optimistic-lock="true|false" generated="never|insert|always" node="element-name|@attribute-name|element/@attribute|." index="index_name" unique_key="unique_key_id" length="L" precision="P" scale="S"/>

name: the name of the property, with an initial lowercase letter.

column (optional - defaults to the property name): the name of the mapped database table column. This can also

be specified by nested <column> element(s).

type (optional): a name that indicates the Hibernate type.

update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE

and/or INSERT statements. Setting both to false allows a pure "derived" property whose value is initialized from

some other property that maps to the same column(s), or by a trigger or other application.

formula (optional): an SQL expression that defines the value for a computed property. Computed properties do not

have a column mapping of their own.


lazy (optional - defaults to false): specifies that this property should be fetched lazily when the instance variable is

first accessed. It requires build-time bytecode instrumentation.

unique (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to be the

target of a property-ref.

not-null (optional): enables the DDL generation of a nullability constraint for the columns.

optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of

the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.

generated (optional - defaults to never): specifies that this property value is actually generated by the database. See

the discussion of generated properties for more information.

typename could be:

1. The name of a Hibernate basic type: integer, string, character, date, timestamp, float, binary, serializable, object, blob etc.

2. The name of a Java class with a default basic type: int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.

3. The name of a serializable Java class.

4. The class name of a custom type: com.illflow.type.MyCustomType etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type.Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. Incertain cases you will need the type attribute. For example, to distinguish between Hibernate.DATE and

Hibernate.TIMESTAMP, or to specify a custom type.

The access attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call

the property get/set pair. If you specify access="field", Hibernate will bypass the get/set pair and access the field directly

using reflection. You can specify your own strategy for property access by naming a class that implements the interfaceorg.hibernate.property.PropertyAccessor.

A powerful feature is derived properties. These properties are by definition read-only. The property value is computed atload time. You declare the computation as an SQL expression. This then translates to a SELECT clause subquery in the

SQL query that loads an instance:

<property name="totalPrice" formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p WHERE li.productId = p.productId AND li.customerId = customerId AND li.orderNumber = orderNumber )"/>

You can reference the entity table by not declaring an alias on a particular column. This would be customerId in the given

example. You can also use the nested <formula> mapping element if you do not want to use the attribute.

5.1.5. Embedded objects (aka components)

Embeddable objects (or components) are objects whose properties are mapped to the same table as the owning entity'stable. Components can, in turn, declare their own properties, components or collections

It is possible to declare an embedded component inside an entity and even override its column mapping. Componentclasses have to be annotated at the class level with the @Embeddable annotation. It is possible to override the column

mapping of an embedded object for a particular entity using the @Embedded and @AttributeOverride annotation in the

associated property:

@Entitypublic class Person implements Serializable {

// Persistent component using defaults Address homeAddress;

@Embedded @AttributeOverrides( { @AttributeOverride(name="iso2", column = @Column(name="bornIso2") ), @AttributeOverride(name="name", column = @Column(name="bornCountryName") ) } ) Country bornIn; ...}

@Embeddablepublic class Address implements Serializable { String city; Country nationality; //no overriding here}

@Embeddablepublic class Country implements Serializable { private String iso2; @Column(name="countryName") private String name;

public String getIso2() { return iso2; } public void setIso2(String iso2) { this.iso2 = iso2; }

public String getName() { return name; } public void setName(String name) { this.name = name; } ...}

An embeddable object inherits the access type of its owning entity (note that you can override that using the @Access

annotation).

The Person entity has two component properties, homeAddress and bornIn. homeAddress property has not been annotated,

but Hibernate will guess that it is a persistent component by looking for the @Embeddable annotation in the Address class.

W e also override the mapping of a column name (to bornCountryName) with the @Embedded and

@AttributeOverride annotations for each mapped attribute of Country. As you can see, Country is also a nested component of

Address, again using auto-detection by Hibernate and JPA defaults. Overriding columns of embedded objects of

embedded objects is through dotted expressions.

@Embedded @AttributeOverrides( { @AttributeOverride(name="city", column = @Column(name="fld_city") ), @AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ), @AttributeOverride(name="nationality.name", column = @Column(name="nat_CountryName") ) //nationality columns in homeAddress are overridden } ) Address homeAddress;

Hibernate Annotations supports something that is not explicitly supported by the JPA specification. You can annotate aembedded object with the @MappedSuperclass annotation to make the superclass properties persistent (see

@MappedSuperclass for more informations).

You can also use association annotations in an embeddable object ( i e @OneToOne, @ManyToOne, @OneToMany or

@ManyToMany). To override the association columns you can use @AssociationOverride.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work asseveral embedded objects would share the same set of columns. In plain JPA, you need to override at least one set ofcolumns. Hibernate, however, allows you to enhance the default naming mechanism through the NamingStrategy interface.

You can write a strategy that prevent name clashing in such a situation. DefaultComponentSafeNamingStrategy is an example

of this.

If a property of the embedded object points back to the owning entity, annotate it with the @Parent annotation. Hibernate

will make sure this property is properly loaded with the entity reference.

In XML, use the <component> element.

<component name="propertyName" class="className" insert="true|false" update="true|false" access="field|property|ClassName" lazy="true|false" optimistic-lock="true|false" unique="true|false" node="element-name|.">

<property ...../> <many-to-one .... /> ........</component>

name: the name of the property.

class (optional - defaults to the property type determined by reflection): the name of the component (child) class.

insert: do the mapped columns appear in SQL INSERTs?

update: do the mapped columns appear in SQL UPDATEs?


lazy (optional - defaults to false): specifies that this component should be fetched lazily when the instance variable

is first accessed. It requires build-time bytecode instrumentation.

optimistic-lock (optional - defaults to true): specifies that updates to this component either do or do not require

acquisition of the optimistic lock. It determines if a version increment should occur when this property is dirty.

unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the

component.

The child <property> tags map properties of the child class to table columns.

The <component> element allows a <parent> subelement that maps a property of the component class as a reference back

to the containing entity.

The <dynamic-component> element allows a Map to be mapped as a component, where the property names refer to keys of

the map. See Section 9.5, “Dynamic components” for more information. This feature is not supported in annotations.

5.1.6. Inheritance strategy

Java is a language supporting polymorphism: a class can inherit from another. Several strategies are possible to persist aclass hierarchy:

Single table per class hierarchy strategy: a single table hosts all the instances of a class hierarchy

Joined subclass strategy: one table per class and subclass is present and each table persist the properties specific toa given subclass. The state of the entity is then stored in its corresponding class table and all its superclasses

Table per class strategy: one table per concrete class and subclass is present and each table persist the properties ofthe class and its superclasses. The state of the entity is then stored entirely in the dedicated table for its class.

5.1.6.1. Single table per class hierarchy strategy

With this approach the properties of all the subclasses in a given mapped class hierarchy are stored in a single table.

Each subclass declares its own persistent properties and subclasses. Version and id properties are assumed to beinherited from the root class. Each subclass in a hierarchy must define a unique discriminator value. If this is not specified,the fully qualified Java class name is used.

@Entity@Inheritance(strategy=InheritanceType.SINGLE_TABLE)@DiscriminatorColumn( name="planetype", discriminatorType=DiscriminatorType.STRING)@DiscriminatorValue("Plane")public class Plane { ... }

@Entity@DiscriminatorValue("A320")public class A320 extends Plane { ... }

In hbm.xml, for the table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. For example:

<subclass name="ClassName" discriminator-value="discriminator_value" proxy="ProxyInterface" lazy="true|false" dynamic-update="true|false" dynamic-insert="true|false" entity-name="EntityName" node="element-name" extends="SuperclassName">

<property .... /> .....</subclass>

name: the fully qualified class name of the subclass.

discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses.

proxy (optional): specifies a class or interface used for lazy initializing proxies.

lazy (optional - defaults to true): setting lazy="false" disables the use of lazy fetching.

For information about inheritance mappings see Chapter 10, Inheritance mapping.

5.1.6.1.1. Discriminator

Discriminators are required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declaresa discriminator column of the table. The discriminator column contains marker values that tell the persistence layer whatsubclass to instantiate for a particular row. Hibernate Core supports the follwoing restricted set of types as discriminatorcolumn: string, character, integer, byte, short, boolean, yes_no, true_false.

Use the @DiscriminatorColumn to define the discriminator column as well as the discriminator type.

Note

The enum DiscriminatorType used in javax.persitence.DiscriminatorColumn only contains the

values STRING, CHAR and INTEGER which means that not all Hibernate supported types

are available via the @DiscriminatorColumn annotation.

You can also use @DiscriminatorFormula to express in SQL a virtual discriminator column. This is particularly useful when

t h e discriminator value can be extracted from one or more columns of the table. Both @DiscriminatorColumn and

@DiscriminatorFormula are to be set on the root entity (once per persisted hierarchy).

@org.hibernate.annotations.DiscriminatorOptions allows to optionally specify Hibernate specific discriminator options which are

not standardized in JPA. The available options are force and insert. The force attribute is useful if the table contains rows

with "extra" discriminator values that are not mapped to a persistent class. This could for example occur when working witha legacy database. If force is set to true Hibernate will specify the allowed discriminator values in the SELECT query, even

when retrieving all instances of the root class. The second option - insert - tells Hibernate whether or not to include the

discriminator column in SQL INSERTs. Usually the column should be part of the INSERT statement, but if your discriminator

column is also part of a mapped composite identifier you have to set this option to false.

Tip

There is also a @org.hibernate.annotations.ForceDiscriminator annotation which is

deprecated since version 3.6. Use @DiscriminatorOptions instead.

Finally, use @DiscriminatorValue on each class of the hierarchy to specify the value stored in the discriminator column for a

given entity. If you do not set @DiscriminatorValue on a class, the fully qualified class name is used.

@Entity@Inheritance(strategy=InheritanceType.SINGLE_TABLE)@DiscriminatorColumn( name="planetype", discriminatorType=DiscriminatorType.STRING)@DiscriminatorValue("Plane")public class Plane { ... }

@Entity@DiscriminatorValue("A320")public class A320 extends Plane { ... }

In hbm.xml, the <discriminator> element is used to define the discriminator column or formula:

<discriminator column="discriminator_column" type="discriminator_type" force="true|false" insert="true|false" formula="arbitrary sql expression" />

column (optional - defaults to class): the name of the discriminator column.

type (optional - defaults to string): a name that indicates the Hibernate type

force (optional - defaults to false): "forces" Hibernate to specify the allowed discriminator values, even when

retrieving all instances of the root class.

insert (optional - defaults to true): set this to false if your discriminator column is also part of a mapped composite

identifier. It tells Hibernate not to include the column in SQL INSERTs.

formula (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It allows content-

based discrimination.

Actual values of the discriminator column are specified by the discriminator-value attribute of the <class> and <subclass>

elements.

The formula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For

example:

<discriminator formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end" type="integer"/>

5.1.6.2. Joined subclass strategy

Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inheritedstate is retrieved by joining with the table of the superclass. A discriminator column is not required for this mappingstrategy. Each subclass must, however, declare a table column holding the object identifier. The primary key of this table isalso a foreign key to the superclass table and described by the @PrimaryKeyJoinColumns or the <key> element.

@Entity @Table(name="CATS")@Inheritance(strategy=InheritanceType.JOINED)public class Cat implements Serializable { @Id @GeneratedValue(generator="cat-uuid") @GenericGenerator(name="cat-uuid", strategy="uuid") String getId() { return id; }

...}

@Entity @Table(name="DOMESTIC_CATS")@PrimaryKeyJoinColumn(name="CAT")public class DomesticCat extends Cat { public String getName() { return name; }}

Note

The table name still defaults to the non qualified class name. Also if@PrimaryKeyJoinColumn is not set, the primary key / foreign key columns are assumed to

have the same names as the primary key columns of the primary table of the superclass.

In hbm.xml, use the <joined-subclass> element. For example:

<joined-subclass name="ClassName" table="tablename" proxy="ProxyInterface" lazy="true|false" dynamic-update="true|false" dynamic-insert="true|false" schema="schema" catalog="catalog" extends="SuperclassName" persister="ClassName" subselect="SQL expression" entity-name="EntityName" node="element-name">

<key .... >

<property .... /> .....</joined-subclass>

table: the name of the subclass table.

proxy (optional): specifies a class or interface to use for lazy initializing proxies.

lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.

Use the <key> element to declare the primary key / foreign key column. The mapping at the start of the chapter would then

be re-written as:

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">


<class name="Cat" table="CATS"> <id name="id" column="uid" type="long"> <generator class="hilo"/> </id> <property name="birthdate" type="date"/> <property name="color" not-null="true"/> <property name="sex" not-null="true"/> <property name="weight"/> <many-to-one name="mate"/> <set name="kittens"> <key column="MOTHER"/> <one-to-many class="Cat"/> </set> <joined-subclass name="DomesticCat" table="DOMESTIC_CATS"> <key column="CAT"/> <property name="name" type="string"/> </joined-subclass> </class>

<class name="eg.Dog">  </class>



5.1.6.3. Table per class strategy

A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it isnot necessary to explicitly map such inheritance hierarchies. You can map each class as a separate entity root. However,if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use theunion subclass mapping.

@Entity@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)public class Flight implements Serializable { ... }

Or in hbm.xml:

<union-subclass name="ClassName" table="tablename" proxy="ProxyInterface" lazy="true|false" dynamic-update="true|false" dynamic-insert="true|false" schema="schema" catalog="catalog" extends="SuperclassName" abstract="true|false" persister="ClassName" subselect="SQL expression" entity-name="EntityName" node="element-name">

<property .... /> .....</union-subclass>


table: the name of the subclass table.

proxy (optional): specifies a class or interface to use for lazy initializing proxies.

lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.

No discriminator column or key column is required for this mapping strategy.


5.1.6.4. Inherit properties from superclasses

This is sometimes useful to share common properties through a technical or a business superclass without including it asa regular mapped entity (ie no specific table for this entity). For that purpose you can map them as @MappedSuperclass.

@MappedSuperclasspublic class BaseEntity { @Basic @Temporal(TemporalType.TIMESTAMP) public Date getLastUpdate() { ... } public String getLastUpdater() { ... } ...}

@Entity class Order extends BaseEntity { @Id public Integer getId() { ... } ...}

In database, this hierarchy will be represented as an Order table having the id, lastUpdate and lastUpdater columns. The

embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddablesuperclass is not the root of the hierarchy though.

Note

Properties from superclasses not mapped as @MappedSuperclass are ignored.

Note

The default access type (field or methods) is used, unless you use the @Access

annotation.

Note

The same notion can be applied to @Embeddable objects to persist properties from their

superclasses. You also need to use @MappedSuperclass to do that (this should not be

considered as a standard EJB3 feature though)

Note

It is allowed to mark a class as @MappedSuperclass in the middle of the mapped

inheritance hierarchy.

Note

Any class in the hierarchy non annotated with @MappedSuperclass nor @Entity will be

ignored.

You can override columns defined in entity superclasses at the root entity level using the @AttributeOverride annotation.

@MappedSuperclasspublic class FlyingObject implements Serializable {

public int getAltitude() { return altitude; }

@Transient public int getMetricAltitude() { return metricAltitude; }

@ManyToOne public PropulsionType getPropulsion() { return metricAltitude; } ...}

@Entity@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )@AssociationOverride( name="propulsion", joinColumns = @JoinColumn(name="fld_propulsion_fk") )public class Plane extends FlyingObject { ...}

Th e altitude property will be persisted in an fld_altitude column of table Plane and the propulsion association will be

materialized in a fld_propulsion_fk foreign key column.

You can define @AttributeOverride(s) and @AssociationOverride(s) on @Entity classes, @MappedSuperclass classes and

properties pointing to an @Embeddable object.

In hbm.xml, simply map the properties of the superclass in the <class> element of the entity that needs to inherit them.

5.1.6.5. Mapping one entity to several tables

While not recommended for a fresh schema, some legacy databases force your to map a single entity on several tables.

Using the @SecondaryTable or @SecondaryTables class level annotations. To express that a column is in a particular table,

use the table parameter of @Column or @JoinColumn.

@Entity@Table(name="MainCat")@SecondaryTables({ @SecondaryTable(name="Cat1", pkJoinColumns={ @PrimaryKeyJoinColumn(name="cat_id", referencedColumnName="id") ), @SecondaryTable(name="Cat2", uniqueConstraints={@UniqueConstraint(columnNames={"storyPart2"})})})public class Cat implements Serializable {

private Integer id; private String name; private String storyPart1; private String storyPart2;

@Id @GeneratedValue public Integer getId() { return id; }

public String getName() { return name; } @Column(table="Cat1") public String getStoryPart1() { return storyPart1; }

@Column(table="Cat2") public String getStoryPart2() { return storyPart2; }}

In this example, name will be in MainCat. storyPart1 will be in Cat1 and storyPart2 will be in Cat2. Cat1 will be joined to MainCat

using the cat_id as a foreign key, and Cat2 using id (ie the same column name, the MainCat id column has). Plus a unique

constraint on storyPart2 has been set.

There is also additional tuning accessible via the @org.hibernate.annotations.Table annotation:

fetch: If set to JOIN, the default, Hibernate will use an inner join to retrieve a secondary table defined by a class or its

superclasses and an outer join for a secondary table defined by a subclass. If set to SELECT then Hibernate will use a

sequential select for a secondary table defined on a subclass, which will be issued only if a row turns out to representan instance of the subclass. Inner joins will still be used to retrieve a secondary defined by the class and itssuperclasses.

inverse: If true, Hibernate will not try to insert or update the properties defined by this join. Default to false.

optional: If enabled (the default), Hibernate will insert a row only if the properties defined by this join are non-null and

will always use an outer join to retrieve the properties.

foreignKey: defines the Foreign Key name of a secondary table pointing back to the primary table.

Make sure to use the secondary table name in the appliesto property

@Entity@Table(name="MainCat")@SecondaryTable(name="Cat1")@org.hibernate.annotations.Table( appliesTo="Cat1", fetch=FetchMode.SELECT, optional=true)public class Cat implements Serializable {

private Integer id; private String name; private String storyPart1; private String storyPart2;

@Id @GeneratedValue public Integer getId() { return id; }

public String getName() { return name; } @Column(table="Cat1") public String getStoryPart1() { return storyPart1; }

@Column(table="Cat2") public String getStoryPart2() { return storyPart2; }}

In hbm.xml, use the <join> element.

<join table="tablename" schema="owner" catalog="catalog" fetch="join|select" inverse="true|false" optional="true|false">

<key ... />

<property ... /> ...</join>

table: the name of the joined table.

schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.

catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.

fetch (optional - defaults to join): if set to join, the default, Hibernate will use an inner join to retrieve a <join> defined

by a class or its superclasses. It will use an outer join for a <join> defined by a subclass. If set to select then

Hibernate will use a sequential select for a <join> defined on a subclass. This will be issued only if a row

represents an instance of the subclass. Inner joins will still be used to retrieve a <join> defined by the class and its

superclasses.

inverse (optional - defaults to false): if enabled, Hibernate will not insert or update the properties defined by this

join.

optional (optional - defaults to false): if enabled, Hibernate will insert a row only if the properties defined by this join

are non-null. It will always use an outer join to retrieve the properties.

For example, address information for a person can be mapped to a separate table while preserving value type semanticsfor all properties:

<class name="Person" table="PERSON">

<id name="id" column="PERSON_ID">...</id>

<join table="ADDRESS"> <key column="ADDRESS_ID"/> <property name="address"/> <property name="zip"/> <property name="country"/> </join> ...

This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-graineddomain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, asexplained later.

5.1.7. Mapping one to one and one to many associations

To link one entity to an other, you need to map the association property as a to one association. In the relational model,you can either use a foreign key or an association table, or (a bit less common) share the same primary key valuebetween the two entities.

To mark an association, use either @ManyToOne or @OnetoOne.

@ManyToOne and @OneToOne have a parameter named targetEntity which describes the target entity name. You usually

don't need this parameter since the default value (the type of the property that stores the association) is good in almost allcases. However this is useful when you want to use interfaces as the return type instead of the regular entity.

Setting a value of the cascade attribute to any meaningful value other than nothing will propagate certain operations to the

associated object. The meaningful values are divided into three categories.

1. basic operations, which include: persist, merge, delete, save-update, evict, replicate, lock and refresh;

2. special values: delete-orphan or all ;

3. comma-separated combinations of operation names: cascade="persist,merge,evict" or cascade="all,delete-orphan".

S e e Section 11.11, “Transitive persistence” for a full explanation. Note that single valued many-to-oneassociations do not support orphan delete.

By default, single point associations are eagerly fetched in JPA 2. You can mark it as lazily fetched by using@ManyToOne(fetch=FetchType.LAZY) in which case Hibernate will proxy the association and load it when the state of the

associated entity is reached. You can force Hibernate not to use a proxy by using @LazyToOne(NO_PROXY). In this case,

t h e property is fetched lazily when the instance variable is first accessed. This requires build-time bytecodeinstrumentation. lazy="false" specifies that the association will always be eagerly fetched.

With the default JPA options, single-ended associations are loaded with a subsequent select if set to LAZY, or a SQL JOIN

is used for EAGER associations. You can however adjust the fetching strategy, ie how data is fetched by using @Fetch.

FetchMode can be SELECT (a select is triggered when the association needs to be loaded) or JOIN (use a SQL JOIN to load

the association while loading the owner entity). JOIN overrides any lazy attribute (an association loaded through a JOIN

strategy cannot be lazy).

5.1.7.1. Using a foreign key or an association table

An ordinary association to another persistent class is declared using a

@ManyToOne if several entities can point to the the target entity

@OneToOne if only a single entity can point to the the target entity

and a foreign key in one table is referencing the primary key column(s) of the target table.

@Entitypublic class Flight implements Serializable { @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} ) @JoinColumn(name="COMP_ID") public Company getCompany() { return company; } ...}

The @JoinColumn attribute is optional, the default value(s) is the concatenation of the name of the relationship in the owner

side, _ (underscore), and the name of the primary key column in the owned side. In this example company_id because the

property name is company and the column id of Company is id.

@Entitypublic class Flight implements Serializable { @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=CompanyImpl.class ) @JoinColumn(name="COMP_ID") public Company getCompany() { return company; } ...}

public interface Company { ...}

You can also map a to one association through an association table. This association table described by the @JoinTable

annotation will contains a foreign key referencing back the entity table (through @JoinTable.joinColumns) and a a foreign key

referencing the target entity table (through @JoinTable.inverseJoinColumns).

@Entitypublic class Flight implements Serializable { @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} ) @JoinTable(name="Flight_Company", joinColumns = @JoinColumn(name="FLIGHT_ID"), inverseJoinColumns = @JoinColumn(name="COMP_ID") ) public Company getCompany() { return company; } ...}

Note

You can use a SQL fragment to simulate a physical join column using the@JoinColumnOrFormula / @JoinColumnOrformulas annotations (just like you can use a SQL

fragment to simulate a property column via the @Formula annotation).

@Entitypublic class Ticket implements Serializable { @ManyToOne @JoinColumnOrFormula(formula="(firstname + ' ' + lastname)") public Person getOwner() { return person; } ...}

You can mark an association as mandatory by using the optional=false attribute. We recommend to use Bean Validation's

@NotNull annotation as a better alternative however. As a consequence, the foreign key column(s) will be marked as not

nullable (if possible).

When Hibernate cannot resolve the association because the expected associated element is not in database (wrong id onthe association column), an exception is raised. This might be inconvenient for legacy and badly maintained schemas.You can ask Hibernate to ignore such elements instead of raising an exception using the @NotFound annotation.

Example 5.1. @NotFound annotation

@Entitypublic class Child { ... @ManyToOne @NotFound(action=NotFoundAction.IGNORE) public Parent getParent() { ... } ...}

Sometimes you want to delegate to your database the deletion of cascade when a given entity is deleted. In this caseHibernate generates a cascade delete constraint at the database level.

Example 5.2. @OnDelete annotation

@Entitypublic class Child { ... @ManyToOne @OnDelete(action=OnDeleteAction.CASCADE) public Parent getParent() { ... } ...}

Foreign key constraints, while generated by Hibernate, have a fairly unreadable name. You can override the constraintname using @ForeignKey.

Example 5.3. @ForeignKey annotation

@Entitypublic class Child { ... @ManyToOne @ForeignKey(name="FK_PARENT") public Parent getParent() { ... } ...}

alter table Child add constraint FK_PARENT foreign key (parent_id) references Parent

Sometimes, you want to link one entity to an other not by the target entity primary key but by a different unique key. Youcan achieve that by referencing the unique key column(s) in @JoinColumn.referenceColumnName.

@Entityclass Person { @Id Integer personNumber; String firstName; @Column(name="I") String initial; String lastName;}

@Entityclass Home { @ManyToOne @JoinColumns({ @JoinColumn(name="first_name", referencedColumnName="firstName"), @JoinColumn(name="init", referencedColumnName="I"), @JoinColumn(name="last_name", referencedColumnName="lastName"), }) Person owner}

This is not encouraged however and should be reserved to legacy mappings.

In hbm.xml, mapping an association is similar. The main difference is that a @OneToOne is mapped as

<many-to-one unique="true"/>, let's dive into the subject.

<many-to-one name="propertyName" column="column_name" class="ClassName" cascade="cascade_style" fetch="join|select" update="true|false" insert="true|false" property-ref="propertyNameFromAssociatedClass" access="field|property|ClassName" unique="true|false" not-null="true|false" optimistic-lock="true|false" lazy="proxy|no-proxy|false" not-found="ignore|exception" entity-name="EntityName" formula="arbitrary SQL expression" node="element-name|@attribute-name|element/@attribute|." embed-xml="true|false" index="index_name" unique_key="unique_key_id" foreign-key="foreign_key_name"/>


column (optional): the name of the foreign key column. This can also be specified by nested <column> element(s).

class (optional - defaults to the property type determined by reflection): the name of the associated class.

cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.

fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.

update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE

and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from

another property that maps to the same column(s), or by a trigger or other application.

property-ref (optional): the name of a property of the associated class that is joined to this foreign key. If not

specified, the primary key of the associated class is used.


unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to

be the target of a property-ref, you can make the association multiplicity one-to-one.

not-null (optional): enables the DDL generation of a nullability constraint for the foreign key columns.

optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of

the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.

lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the

property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecodeinstrumentation. lazy="false" specifies that the association will always be eagerly fetched.

not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows will be handled.

ignore will treat a missing row as a null association.

entity-name (optional): the entity name of the associated class.

formula (optional): an SQL expression that defines the value for a computed foreign key.

Setting a value of the cascade attribute to any meaningful value other than none will propagate certain operations to the

associated object. The meaningful values are divided into three categories. First, basic operations, which include: persist, merge, delete, save-update, evict, replicate, lock and refresh; second, special values: delete-orphan; and third,all comma-

separated combinations of operation names: cascade="persist,merge,evict" or cascade="all,delete-orphan". See Section 11.11,

“Transitive persistence” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do notsupport orphan delete.

Here is an example of a typical many-to-one declaration:

<many-to-one name="product" class="Product" column="PRODUCT_ID"/>

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the

associated table other than the primary key. This is a complicated and confusing relational model. For example, if theProduct class had a unique serial number that is not the primary key. The unique attribute controls Hibernate's DDL

generation with the SchemaExport tool.

<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>

Then the mapping for OrderItem might use:

<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>

This is not encouraged, however.

If the referenced unique key comprises multiple properties of the associated entity, you should map the referencedproperties inside a named <properties> element.

If the referenced unique key is the property of a component, you can specify a property path:

<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>

5.1.7.2. Sharing the primary key with the associated entity

The second approach is to ensure an entity and its associated entity share the same primary key. In this case the primarykey column is also a foreign key and there is no extra column. These associations are always one to one.

Example 5.4. One to One association

@Entitypublic class Body { @Id public Long getId() { return id; }

@OneToOne(cascade = CascadeType.ALL) @MapsId public Heart getHeart() { return heart; } ...}

@Entitypublic class Heart { @Id public Long getId() { ...}}

Note

Many people got confused by these primary key based one to one associations. Theycan only be lazily loaded if Hibernate knows that the other side of the association isalways present. To indicate to Hibernate that it is the case, use@OneToOne(optional=false).

In hbm.xml, use the following mapping.

<one-to-one name="propertyName" class="ClassName" cascade="cascade_style" constrained="true|false" fetch="join|select" property-ref="propertyNameFromAssociatedClass" access="field|property|ClassName" formula="any SQL expression" lazy="proxy|no-proxy|false" entity-name="EntityName" node="element-name|@attribute-name|element/@attribute|." embed-xml="true|false" foreign-key="foreign_key_name"/>


class (optional - defaults to the property type determined by reflection): the name of the associated class.

cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.

constrained (optional): specifies that a foreign key constraint on the primary key of the mapped table and

references the table of the associated class. This option affects the order in which save() and delete() are

cascaded, and determines whether the association can be proxied. It is also used by the schema export tool.

fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.

property-ref (optional): the name of a property of the associated class that is joined to the primary key of this class.

If not specified, the primary key of the associated class is used.


formula (optional): almost all one-to-one associations map to the primary key of the owning entity. If this is not the

case, you can specify another column, columns or expression to join on using an SQL formula. Seeorg.hibernate.test.onetooneformula for an example.

lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the

property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecodeinstrumentation. lazy="false" specifies that the association will always be eagerly fetched. Note that if

constrained="false", proxying is impossible and Hibernate will eagerly fetch the association.

entity-name (optional): the entity name of the associated class.

Primary key associations do not need an extra table column. If two rows are related by the association, then the two tablerows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned

the same identifier value.

For a primary key association, add the following mappings to Employee and Person respectively:

<one-to-one name="person" class="Person"/>

<one-to-one name="employee" class="Employee" constrained="true"/>

Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a specialHibernate identifier generation strategy called foreign:

<class name="person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="foreign"> <param name="property">employee</param> </generator> </id> ... <one-to-one name="employee" class="Employee" constrained="true"/></class>

A newly saved instance of Person is assigned the same primary key value as the Employee instance referred with the

employee property of that Person.

5.1.8. Natural-id

Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. Anatural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the propertiesof the natural key as @NaturalId or map them inside the <natural-id> element. Hibernate will generate the necessary unique

key and nullability constraints and, as a result, your mapping will be more self-documenting.

@Entitypublic class Citizen { @Id @GeneratedValue private Integer id; private String firstname; private String lastname; @NaturalId @ManyToOne private State state;

@NaturalId private String ssn; ...}

//and later on queryList results = s.createCriteria( Citizen.class ) .add( Restrictions.naturalId().set( "ssn", "1234" ).set( "state", ste ) ) .list();

Or in XML,

<natural-id mutable="true|false"/> <property ... /> <many-to-one ... /> ......</natural-id>

It is recommended that you implement equals() and hashCode() to compare the natural key properties of the entity.

This mapping is not intended for use with entities that have natural primary keys.

mutable (optional - defaults to false): by default, natural identifier properties are assumed to be immutable (constant).

5.1.9. Any

There is one more type of property mapping. The @Any mapping defines a polymorphic association to classes from

multiple tables. This type of mapping requires more than one column. The first column contains the type of the associatedentity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind ofassociation. This is not the usual way of mapping polymorphic associations and you should use this only in special cases.For example, for audit logs, user session data, etc.

Th e @Any annotation describes the column holding the metadata information. To link the value of the metadata

information and an actual entity type, The @AnyDef and @AnyDefs annotations are used. The metaType attribute allows the

application to specify a custom type that maps database column values to persistent classes that have identifier propertiesof the type specified by idType. You must specify the mapping from values of the metaType to class names.

@Any( metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )@AnyMetaDef( idType = "integer", metaType = "string", metaValues = { @MetaValue( value = "S", targetEntity = StringProperty.class ), @MetaValue( value = "I", targetEntity = IntegerProperty.class ) } )@JoinColumn( name = "property_id" )public Property getMainProperty() { return mainProperty;}

Note that @AnyDef can be mutualized and reused. It is recommended to place it as a package metadata in this case.

//on a package@AnyMetaDef( name="property" idType = "integer", metaType = "string", metaValues = { @MetaValue( value = "S", targetEntity = StringProperty.class ), @MetaValue( value = "I", targetEntity = IntegerProperty.class ) } )package org.hibernate.test.annotations.any;

//in a class @Any( metaDef="property", metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER ) @JoinColumn( name = "property_id" ) public Property getMainProperty() { return mainProperty; }

The hbm.xml equivalent is:

<any name="being" id-type="long" meta-type="string"> <meta-value value="TBL_ANIMAL" class="Animal"/> <meta-value value="TBL_HUMAN" class="Human"/> <meta-value value="TBL_ALIEN" class="Alien"/> <column name="table_name"/> <column name="id"/></any>

Note

You cannot mutualize the metadata in hbm.xml as you can in annotations.

<any name="propertyName" id-type="idtypename" meta-type="metatypename" cascade="cascade_style" access="field|property|ClassName" optimistic-lock="true|false" > <meta-value ... /> <meta-value ... /> ..... <column .... /> <column .... /> .....</any>

name: the property name.

id-type: the identifier type.

meta-type (optional - defaults to string): any type that is allowed for a discriminator mapping.

cascade (optional- defaults to none): the cascade style.


optimistic-lock (optional - defaults to true): specifies that updates to this property either do or do not require

acquisition of the optimistic lock. It defines whether a version increment should occur if this property is dirty.

5.1.10. Properties

The <properties> element allows the definition of a named, logical grouping of the properties of a class. The most important

use of the construct is that it allows a combination of properties to be the target of a property-ref. It is also a convenient way

to define a multi-column unique constraint. For example:

<properties name="logicalName" insert="true|false" update="true|false" optimistic-lock="true|false" unique="true|false" >

<property ...../> <many-to-one .... /> ........</properties>

name: the logical name of the grouping. It is not an actual property name.

insert: do the mapped columns appear in SQL INSERTs?

update: do the mapped columns appear in SQL UPDATEs?

optimistic-lock (optional - defaults to true): specifies that updates to these properties either do or do not require

acquisition of the optimistic lock. It determines if a version increment should occur when these properties are dirty.

unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the

component.

For example, if we have the following <properties> mapping:

<class name="Person"> <id name="personNumber"/>

... <properties name="name" unique="true" update="false"> <property name="firstName"/> <property name="initial"/> <property name="lastName"/> </properties></class>

You might have some legacy data association that refers to this unique key of the Person table, instead of to the primary

key:

<many-to-one name="owner" class="Person" property-ref="name"> <column name="firstName"/> <column name="initial"/> <column name="lastName"/></many-to-one>

Note

When using annotations as a mapping strategy, such construct is not necessary as thebinding between a column and its related column on the associated table is done directly

@Entityclass Person { @Id Integer personNumber; String firstName; @Column(name="I") String initial; String lastName;}

@Entityclass Home { @ManyToOne @JoinColumns({ @JoinColumn(name="first_name", referencedColumnName="firstName"), @JoinColumn(name="init", referencedColumnName="I"), @JoinColumn(name="last_name", referencedColumnName="lastName"), }) Person owner}

The use of this outside the context of mapping legacy data is not recommended.

5.1.11. Some hbm.xml specificities

The hbm.xml structure has some specificities naturally not present when using annotations, let's describe them briefly.

5.1.11.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directoryhibernate-x.x.x/src/org/hibernate , or in hibernate3.jar. Hibernate will always look for the DTD in its classpath first. If you

experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of yourclasspath.

5.1.11.1.1. EntityResolver

Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a custom org.xml.sax.EntityResolver

implementation with the SAXReader it uses to read in the xml files. This custom EntityResolver recognizes two different

systemId namespaces:

a hibernate namespace is recognized whenever the resolver encounters a systemId starting with

http://www.hibernate.org/dtd/. The resolver attempts to resolve these entities via the classloader which loaded the

Hibernate classes.

a user namespace is recognized whenever the resolver encounters a systemId using a classpath:// URL protocol. The

resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloaderwhich loaded the Hibernate classes.

The following is an example of utilizing user namespacing:

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [ <!ENTITY types SYSTEM "classpath://your/domain/types.xml">]>

<hibernate-mapping package="your.domain"> <class name="MyEntity"> <id name="id" type="my-custom-id-type"> ... </id> <class> &types;</hibernate-mapping>

Where types.xml is a resource in the your.domain package and contains a custom typedef.

5.1.11.2. Hibernate-mapping

This element has several optional attributes. The schema and catalog attributes specify that tables referred to in this

mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the givenschema and catalog names. If they are missing, tablenames will be unqualified. The default-cascade attribute specifies

what cascade style should be assumed for properties and collections that do not specify a cascade attribute. By default, the

auto-import attribute allows you to use unqualified class names in the query language.

<hibernate-mapping schema="schemaName" catalog="catalogName" default-cascade="cascade_style" default-access="field|property|ClassName" default-lazy="true|false" auto-import="true|false" package="package.name" />

schema (optional): the name of a database schema.

catalog (optional): the name of a database catalog.

default-cascade (optional - defaults to none): a default cascade style.

default-access (optional - defaults to property): the strategy Hibernate should use for accessing all properties. It can

be a custom implementation of PropertyAccessor.

default-lazy (optional - defaults to true): the default value for unspecified lazy attributes of class and collection

mappings.

auto-import (optional - defaults to true): specifies whether we can use unqualified class names of classes in this

mapping in the query language.

package (optional): specifies a package prefix to use for unqualified class names in the mapping document.

If you have two persistent classes with the same unqualified name, you should set auto-import="false". An exception will

result if you attempt to assign two classes to the same "imported" name.

The hibernate-mapping element allows you to nest several persistent <class> mappings, as shown above. It is, however,

good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in onemapping file and name it after the persistent superclass. For example, Cat.hbm.xml, Dog.hbm.xml, or if using inheritance,

Animal.hbm.xml.

5.1.11.3. Key

The <key> element is featured a few times within this guide. It appears anywhere the parent mapping element defines a

join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:

<key column="columnname" on-delete="noaction|cascade" property-ref="propertyName" not-null="true|false" update="true|false" unique="true|false" />

column (optional): the name of the foreign key column. This can also be specified by nested <column> element(s).

on-delete (optional - defaults to noaction): specifies whether the foreign key constraint has database-level cascade

delete enabled.

property-ref (optional): specifies that the foreign key refers to columns that are not the primary key of the original

table. It is provided for legacy data.

not-null (optional): specifies that the foreign key columns are not nullable. This is implied whenever the foreign key

is also part of the primary key.

update (optional): specifies that the foreign key should never be updated. This is implied whenever the foreign key

is also part of the primary key.

unique (optional): specifies that the foreign key should have a unique constraint. This is implied whenever the

foreign key is also the primary key.

For systems where delete performance is important, we recommend that all keys should be defined on-delete="cascade".

Hibernate uses a database-level ON CASCADE DELETE constraint, instead of many individual DELETE statements. Be

aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.

Th e not-null and update attributes are useful when mapping a unidirectional one-to-many association. If you map a

unidirectional one-to-many association to a non-nullable foreign key, you must declare the key column using<key not-null="true">.

5.1.11.4. Import

If your application has two persistent classes with the same name, and you do not want to specify the fully qualifiedpackage name in Hibernate queries, classes can be "imported" explicitly, rather than relying upon auto-import="true". You

can also import classes and interfaces that are not explicitly mapped:

<import class="java.lang.Object" rename="Universe"/>

<import class="ClassName" rename="ShortName" />

class: the fully qualified class name of any Java class.

rename (optional - defaults to the unqualified class name): a name that can be used in the query language.

Note

This feature is unique to hbm.xml and is not supported in annotations.

5.1.11.5. Column and formula elements

Mapping elements which accept a column attribute will alternatively accept a <column> subelement. Likewise, <formula> is

an alternative to the formula attribute. For example:

<column name="column_name" length="N" precision="N" scale="N" not-null="true|false" unique="true|false" unique-key="multicolumn_unique_key_name" index="index_name" sql-type="sql_type_name" check="SQL expression" default="SQL expression" read="SQL expression" write="SQL expression"/>

<formula>SQL expression</formula>

Most of the attributes on column provide a means of tailoring the DDL during automatic schema generation. The read and

write attributes allow you to specify custom SQL that Hibernate will use to access the column's value. For more on this, see

the discussion of column read and write expressions.

The column and formula elements can even be combined within the same property or association mapping to express, for

example, exotic join conditions.

<many-to-one name="homeAddress" class="Address" insert="false" update="false"> <column name="person_id" not-null="true" length="10"/> <formula>'MAILING'</formula></many-to-one>


5.2.1. Entities and values

In relation to the persistence service, Java language-level objects are classified into two groups:

A n entity exists independently of any other objects holding references to the entity. Contrast this with the usual Javamodel, where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted. Saves anddeletions, however, can be cascaded from a parent entity to its children. This is different from the ODMG model of objectpersistence by reachability and corresponds more closely to how application objects are usually used in large systems.Entities support circular and shared references. They can also be versioned.

An entity's persistent state consists of references to other entities and instances of value types. Values are primitives:collections (not what is inside a collection), components and certain immutable objects. Unlike entities, values in particularcollections and components, are persisted and deleted by reachability. Since value objects and primitives are persistedand deleted along with their containing entity, they cannot be independently versioned. Values have no independentidentity, so they cannot be shared by two entities or collections.

Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that. Not all user-defined classes with a persistent state, however, are entities. A component is a user-defined class with value semantics. AJava property of type java.lang.String also has value semantics. Given this definition, all types (classes) provided by the JDK

have value type semantics in Java, while user-defined types can be mapped with entity or value type semantics. Thisdecision is up to the application developer. An entity class in a domain model will normally have shared references to asingle instance of that class, while composition or aggregation usually translates to a value type.

We will revisit both concepts throughout this reference guide.

The challenge is to map the Java type system, and the developers' definition of entities and value types, to theSQL/database type system. The bridge between both systems is provided by Hibernate. For entities, <class>, <subclass>

and so on are used. For value types we use <property>, <component>etc., that usually have a type attribute. The value of this

attribute is the name of a Hibernate mapping type. Hibernate provides a range of mappings for standard JDK value typesout of the box. You can write your own mapping types and implement your own custom conversion strategies.

With the exception of collections, all built-in Hibernate types support null semantics.

5.2.2. Basic value types

The built-in basic mapping types can be roughly categorized into the following:

integer, long, short, float, double, character, byte, boolean, yes_no, true_false

Type mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column types.boolean, yes_no and true_false are all alternative encodings for a Java boolean or java.lang.Boolean.

string

A type mapping from java.lang.String to VARCHAR (or Oracle VARCHAR2).

date, time, timestamp

Type mappings from java.util.Date and its subclasses to SQL types DATE, TIME and TIMESTAMP (or equivalent).

calendar, calendar_date

Type mappings from java.util.Calendar to SQL types TIMESTAMP and DATE (or equivalent).

big_decimal, big_integer

Type mappings from java.math.BigDecimal and java.math.BigInteger to NUMERIC (or Oracle NUMBER).

locale, timezone, currency

Type mappings from java.util.Locale, java.util.TimeZone and java.util.Currency to VARCHAR (or Oracle VARCHAR2).

Instances of Locale and Currency are mapped to their ISO codes. Instances of TimeZone are mapped to their ID.

class

A type mapping from java.lang.Class to VARCHAR (or Oracle VARCHAR2). A Class is mapped to its fully qualified name.

binary

Maps byte arrays to an appropriate SQL binary type.

text

Maps long Java strings to a SQL LONGVARCHAR or TEXT type.

image

Maps long byte arrays to a SQL LONGVARBINARY.

serializable

Maps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate type serializable

with the name of a serializable Java class or interface that does not default to a basic type.

clob, blob

Type mappings for the JDBC classes java.sql.Clob and java.sql.Blob. These types can be inconvenient for some

applications, since the blob or clob object cannot be reused outside of a transaction. Driver support is patchy andinconsistent.

materialized_clob

Maps long Java strings to a SQL CLOB type. When read, the CLOB value is immediately materialized into a Java

string. Some drivers require the CLOB value to be read within a transaction. Once materialized, the Java string is

available outside of the transaction.

materialized_blob

Maps long Java byte arrays to a SQL BLOB type. When read, the BLOB value is immediately materialized into a byte

array. Some drivers require the BLOB value to be read within a transaction. Once materialized, the byte array is

available outside of the transaction.

imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binary

Type mappings for what are considered mutable Java types. This is where Hibernate makes certain optimizationsappropriate only for immutable Java types, and the application treats the object as immutable. For example, youshould not call Date.setTime() for an instance mapped as imm_timestamp. To change the value of the property, and have

that change made persistent, the application must assign a new, nonidentical, object to the property.

Unique identifiers of entities and collections can be of any basic type except binary, blob and clob. Composite identifiers are

also allowed. See below for more information.

The basic value types have corresponding Type constants defined on org.hibernate.Hibernate. For example,

Hibernate.STRING represents the string type.

5.2.3. Custom value types

It is relatively easy for developers to create their own value types. For example, you might want to persist properties of typejava.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in type for this. Custom types are not limited to

mapping a property, or collection element, to a single table column. So, for example, you might have a Java propertygetName()/setName() of type java.lang.String that is persisted to the columns FIRST_NAME, INITIAL, SURNAME.

To implement a custom type, implement either org.hibernate.UserType or org.hibernate.CompositeUserType and declare

properties using the fully qualified classname of the type. View org.hibernate.test.DoubleStringType to see the kind of things

that are possible.

<property name="twoStrings" type="org.hibernate.test.DoubleStringType"> <column name="first_string"/> <column name="second_string"/></property>

Notice the use of <column> tags to map a property to multiple columns.

The CompositeUserType, EnhancedUserType, UserCollectionType, and UserVersionType interfaces provide support for more

specialized uses.

You can even supply parameters to a UserType in the mapping file. To do this, your UserType must implement the

org.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the <type>

element in your mapping files.

<property name="priority"> <type name="com.mycompany.usertypes.DefaultValueIntegerType"> <param name="default">0</param> </type></property>

The UserType can now retrieve the value for the parameter named default from the Properties object passed to it.

If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using the <typedef>

element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type isparameterized.

<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero"> <param name="default">0</param></typedef>

<property name="priority" type="default_zero"/>

It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters onthe property mapping.

Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use acustom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your

application. For example, a MonetaryAmount class is a good candidate for a CompositeUserType, even though it could be

mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would beprotected against changes to the way monetary values are represented.

5.3. Mapping a class more than once

It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify an entityname to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the classname. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or whenmapping associations to the named entity.

<class name="Contract" table="Contracts" entity-name="CurrentContract"> ... <set name="history" inverse="true" order-by="effectiveEndDate desc"> <key column="currentContractId"/> <one-to-many entity-name="HistoricalContract"/> </set></class>

<class name="Contract" table="ContractHistory" entity-name="HistoricalContract"> ... <many-to-one name="currentContract" column="currentContractId" entity-name="CurrentContract"/></class>

Associations are now specified using entity-name instead of class.

Note

This feature is not supported in Annotations

5.4. SQL quoted identifiers

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks inthe mapping document. Hibernate will use the correct quotation style for the SQL Dialect. This is usually double quotes, but

the SQL Server uses brackets and MySQL uses backticks.

@Entity @Table(name="`Line Item`")class LineItem { @id @Column(name="Ìtem Id`") Integer id; @Column(name="Ìtem #`") int itemNumber}

<class name="LineItem" table="`Line Item`"> <id name="id" column="Ìtem Id`"/><generator class="assigned"/></id> <property name="itemNumber" column="Ìtem #`"/> ...</class>

5.5. Generated properties

Generated properties are properties that have their values generated by the database. Typically, Hibernate applicationsneeded to refresh objects that contain any properties for which the database was generating values. Marking properties as

generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERTor UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve thegenerated values.

Properties marked as generated must additionally be non-insertable and non-updateable. Only versions, timestamps, andsimple properties, can be marked as generated.

never (the default): the given property value is not generated within the database.

insert: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like

created-date fall into this category. Even though version and timestamp properties can be marked as generated, thisoption is not available.

always: the property value is generated both on insert and on update.

To mark a property as generated, use @Generated.

5.6. Column transformers: read and write expressions

Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped to simple properties.For example, if your database provides a set of data encryption functions, you can invoke them for individual columns likethis:

@Entityclass CreditCard { @Column(name="credit_card_num") @ColumnTransformer( read="decrypt(credit_card_num)", write="encrypt(?)") public String getCreditCardNumber() { return creditCardNumber; } public void setCreditCardNumber(String number) { this.creditCardNumber = number; } private String creditCardNumber;}

or in XML

<property name="creditCardNumber"> <column name="credit_card_num" read="decrypt(credit_card_num)" write="encrypt(?)"/></property>

Note

You can use the plural form @ColumnTransformers if more than one columns need to

define either of these rules.

If a property uses more that one column, you must use the forColumn attribute to specify which column, the expressions are

targeting.

@Entityclass User { @Type(type="com.acme.type.CreditCardType") @Columns( { @Column(name="credit_card_num"), @Column(name="exp_date") } ) @ColumnTransformer( forColumn="credit_card_num", read="decrypt(credit_card_num)", write="encrypt(?)") public CreditCard getCreditCard() { return creditCard; } public void setCreditCard(CreditCard card) { this.creditCard = card; } private CreditCard creditCard;}

Hibernate applies the custom expressions automatically whenever the property is referenced in a query. This functionalityis similar to a derived-property formula with two differences:

The property is backed by one or more columns that are exported as part of automatic schema generation.

The property is read-write, not read-only.

The write expression, if specified, must contain exactly one '?' placeholder for the value.

5.7. Auxiliary database objects

Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate'sschema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Althoughdesigned specifically for creating and dropping things like triggers or stored procedures, any SQL command that can berun via a java.sql.Statement.execute() method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two

modes for defining auxiliary database objects:

The first mode is to explicitly list the CREATE and DROP commands in the mapping file:

<hibernate-mapping> ... <database-object> <create>CREATE TRIGGER my_trigger ...</create> <drop>DROP TRIGGER my_trigger</drop> </database-object></hibernate-mapping>

The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class mustimplement the org.hibernate.mapping.AuxiliaryDatabaseObject interface.

<hibernate-mapping> ... <database-object> <definition class="MyTriggerDefinition"/> </database-object></hibernate-mapping>

Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.

<hibernate-mapping> ... <database-object> <definition class="MyTriggerDefinition"/> <dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/> <dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/> </database-object></hibernate-mapping>

Note

This feature is not supported in Annotations

Chapter 6. Types

Table of Contents

6.1. Value types

6.1.1. Basic value types6.1.2. Composite types6.1.3. Collection types

6.2. Entity types6.3. Significance of type categories6.4. Custom types




6.5. Type registry

As an Object/Relational Mapping solution, Hibernate deals with both the Java and JDBC representations of applicationdata. An online catalog application, for example, most likely has Product object with a number of attributes such as a sku,

name, etc. For these individual attributes, Hibernate must be able to read the values out of the database and write them

back. This 'marshalling' is the function of a Hibernate type, which is an implementation of the org.hibernate.type.Type

interface. In addition, a Hibernate type describes various aspects of behavior of the Java type such as "how is equalitychecked?" or "how are values cloned?".

Important

A Hibernate type is neither a Java type nor a SQL datatype; it provides a informationabout both.When you encounter the term type in regards to Hibernate be aware that usage mightrefer to the Java type, the SQL/JDBC type or the Hibernate type.

Hibernate categorizes types into two high-level groups: value types (see Section 6.1, “Value types”) and entity types (seeSection 6.2, “Entity types”).

6.1. Value types

The main distinguishing characteristic of a value type is the fact that they do not define their own lifecycle. We say that theyare "owned" by something else (specifically an entity, as we will see later) which defines their lifecycle. Value types arefurther classified into 3 sub-categories: basic types (see Section 6.1.1, “Basic value types”), composite types (seeSection 6.1.2, “Composite types”) amd collection types (see Section 6.1.3, “Collection types”).

6.1.1. Basic value types

The norm for basic value types is that they map a single database value (column) to a single, non-aggregated Java type.Hibernate provides a number of built-in basic types, which we will present in the following sections by the Java type.Mainly these follow the natural mappings recommended in the JDBC specification. We will later cover how to overridethese mapping and how to provide and use alternative type mappings.

6.1.1.1. java.lang.String

org.hibernate.type.StringType

Maps a string to the JDBC VARCHAR type. This is the standard mapping for a string if no Hibernate type is specified.

Registered under string and java.lang.String in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.MaterializedClob

Maps a string to a JDBC CLOB type

Registered under materialized_clob in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.TextType

Maps a string to a JDBC LONGVARCHAR type

Registered under text in the type registry (see Section 6.5, “Type registry”).

6.1.1.2. java.lang.Character (or char primitive)

org.hibernate.type.CharacterType

Maps a char or java.lang.Character to a JDBC CHAR

Registered under char and java.lang.Character in the type registry (see Section 6.5, “Type registry”).

6.1.1.3. java.lang.Boolean (or boolean primitive)

org.hibernate.type.BooleanType

Maps a boolean to a JDBC BIT type

Registered under boolean and java.lang.Boolean in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.NumericBooleanType

Maps a boolean to a JDBC INTEGER type as 0 = false, 1 = true

Registered under numeric_boolean in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.YesNoType

Maps a boolean to a JDBC CHAR type as ('N' | 'n') = false, ( 'Y' | 'y' ) = true

Registered under yes_no in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.TrueFalseType

Maps a boolean to a JDBC CHAR type as ('F' | 'f') = false, ( 'T' | 't' ) = true

Registered under true_false in the type registry (see Section 6.5, “Type registry”).

6.1.1.4. java.lang.Byte (or byte primitive)

org.hibernate.type.ByteType

Maps a byte or java.lang.Byte to a JDBC TINYINT

Registered under byte and java.lang.Byte in the type registry (see Section 6.5, “Type registry”).

6.1.1.5. java.lang.Short (or short primitive)

org.hibernate.type.ShortType

Maps a short or java.lang.Short to a JDBC SMALLINT

Registered under short and java.lang.Short in the type registry (see Section 6.5, “Type registry”).

6.1.1.6. java.lang.Integer (or int primitive)

org.hibernate.type.IntegerTypes

Maps an int or java.lang.Integer to a JDBC INTEGER

Registered under int and java.lang.Integerin the type registry (see Section 6.5, “Type registry”).

6.1.1.7. java.lang.Long (or long primitive)

org.hibernate.type.LongType

Maps a long or java.lang.Long to a JDBC BIGINT

Registered under long and java.lang.Long in the type registry (see Section 6.5, “Type registry”).

6.1.1.8. java.lang.Float (or float primitive)

org.hibernate.type.FloatType

Maps a float or java.lang.Float to a JDBC FLOAT

Registered under float and java.lang.Float in the type registry (see Section 6.5, “Type registry”).

6.1.1.9. java.lang.Double (or double primitive)

org.hibernate.type.DoubleType

Maps a double or java.lang.Double to a JDBC DOUBLE

Registered under double and java.lang.Double in the type registry (see Section 6.5, “Type registry”).

6.1.1.10. java.math.BigInteger

org.hibernate.type.BigIntegerType

Maps a java.math.BigInteger to a JDBC NUMERIC

Registered under big_integer and java.math.BigInteger in the type registry (see Section 6.5, “Type registry”).

6.1.1.11. java.math.BigDecimal

org.hibernate.type.BigDecimalType

Maps a java.math.BigDecimal to a JDBC NUMERIC

Registered under big_decimal and java.math.BigDecimal in the type registry (see Section 6.5, “Type registry”).

6.1.1.12. java.util.Date or java.sql.Timestamp

org.hibernate.type.TimestampType

Maps a java.sql.Timestamp to a JDBC TIMESTAMP

Registered under timestamp, java.sql.Timestamp and java.util.Date in the type registry (see Section 6.5, “Type registry”).

6.1.1.13. java.sql.Time

org.hibernate.type.TimeType

Maps a java.sql.Time to a JDBC TIME

Registered under time and java.sql.Time in the type registry (see Section 6.5, “Type registry”).

6.1.1.14. java.sql.Date

org.hibernate.type.DateType

Maps a java.sql.Date to a JDBC DATE

Registered under date and java.sql.Date in the type registry (see Section 6.5, “Type registry”).

6.1.1.15. java.util.Calendar

org.hibernate.type.CalendarType

Maps a java.util.Calendar to a JDBC TIMESTAMP

Registered under calendar, java.util.Calendar and java.util.GregorianCalendar in the type registry (see Section 6.5, “Type

registry”).

org.hibernate.type.CalendarDateType

Maps a java.util.Calendar to a JDBC DATE

Registered under calendar_date in the type registry (see Section 6.5, “Type registry”).

6.1.1.16. java.util.Currency

org.hibernate.type.CurrencyType

Maps a java.util.Currency to a JDBC VARCHAR (using the Currency code)

Registered under currency and java.util.Currency in the type registry (see Section 6.5, “Type registry”).

6.1.1.17. java.util.Locale

org.hibernate.type.LocaleType

Maps a java.util.Locale to a JDBC VARCHAR (using the Locale code)

Registered under locale and java.util.Locale in the type registry (see Section 6.5, “Type registry”).

6.1.1.18. java.util.TimeZone

org.hibernate.type.TimeZoneType

Maps a java.util.TimeZone to a JDBC VARCHAR (using the TimeZone ID)

Registered under timezone and java.util.TimeZone in the type registry (see Section 6.5, “Type registry”).

6.1.1.19. java.net.URL

org.hibernate.type.UrlType

Maps a java.net.URL to a JDBC VARCHAR (using the external form)

Registered under url and java.net.URL in the type registry (see Section 6.5, “Type registry”).

6.1.1.20. java.lang.Class

org.hibernate.type.ClassType

Maps a java.lang.Class to a JDBC VARCHAR (using the Class name)

Registered under class and java.lang.Class in the type registry (see Section 6.5, “Type registry”).

6.1.1.21. java.sql.Blob

org.hibernate.type.BlobType

Maps a java.sql.Blob to a JDBC BLOB

Registered under blob and java.sql.Blob in the type registry (see Section 6.5, “Type registry”).

6.1.1.22. java.sql.Clob

org.hibernate.type.ClobType

Maps a java.sql.Clob to a JDBC CLOB

Registered under clob and java.sql.Clob in the type registry (see Section 6.5, “Type registry”).

6.1.1.23. byte[]

org.hibernate.type.BinaryType

Maps a primitive byte[] to a JDBC VARBINARY

Registered under binary and byte[] in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.MaterializedBlobType

Maps a primitive byte[] to a JDBC BLOB

Registered under materialized_blob in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.ImageType

Maps a primitive byte[] to a JDBC LONGVARBINARY

Registered under image in the type registry (see Section 6.5, “Type registry”).

6.1.1.24. Byte[]

org.hibernate.type.BinaryType

Maps a java.lang.Byte[] to a JDBC VARBINARY

Registered under wrapper-binary, Byte[] and java.lang.Byte[] in the type registry (see Section 6.5, “Type registry”).

6.1.1.25. char[]

org.hibernate.type.CharArrayType

Maps a char[] to a JDBC VARCHAR

Registered under characters and char[] in the type registry (see Section 6.5, “Type registry”).

6.1.1.26. Character[]

org.hibernate.type.CharacterArrayType

Maps a java.lang.Character[] to a JDBC VARCHAR

Registered under wrapper-characters, Character[] and java.lang.Character[] in the type registry (see Section 6.5, “Type

registry”).

6.1.1.27. java.util.UUID

org.hibernate.type.UUIDBinaryType

Maps a java.util.UUID to a JDBC BINARY

Registered under uuid-binary and java.util.UUID in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.UUIDCharType

Maps a java.util.UUID to a JDBC CHAR (though VARCHAR is fine too for existing schemas)

Registered under uuid-char in the type registry (see Section 6.5, “Type registry”).

org.hibernate.type.PostgresUUIDType

Maps a java.util.UUID to the PostgreSQL UUID data type (through Types#OTHER which is how the PostgreSQL JDBC

driver defines it).

Registered under pg-uuid in the type registry (see Section 6.5, “Type registry”).

6.1.1.28. java.io.Serializable

org.hibernate.type.SerializableType

Maps implementors of java.lang.Serializable to a JDBC VARBINARY

Unlike the other value types, there are multiple instances of this type. It gets registered once under java.io.Serializable.

Additionally it gets registered under the specific java.io.Serializable implementation class names.

6.1.2. Composite types

Note

The Java Persistence API calls these embedded types, while Hibernate traditionallycalled them components. Just be aware that both terms are used and mean the samething in the scope of discussing Hibernate.

Components represent aggregations of values into a single Java type. For example, you might have an Address class thataggregates street, city, state, etc information or a Name class that aggregates the parts of a person's Name. In many waysa component looks exactly like an entity. They are both (generally speaking) classes written specifically for the application.They both might have references to other application-specific classes, as well as to collections and simple JDK types. Asdiscussed before, the only distinguishing factory is the fact that a component does not own its own lifecycle nor does itdefine an identifier.

6.1.3. Collection types

Important

It is critical understand that we mean the collection itself, not its contents. The contents ofthe collection can in turn be basic, component or entity types (though not collections), butthe collection itself is owned.

Collections are covered in Chapter 7, Collection mapping.

6.2. Entity types

The definition of entities is covered in detail in Chapter 4, Persistent Classes. For the purpose of this discussion, it isenough to say that entities are (generally application-specific) classes which correlate to rows in a table. Specifically theycorrelate to the row by means of a unique identifier. Because of this unique identifier, entities exist independently anddefine their own lifecycle. As an example, when we delete a Membership, both the User and Group entities remain.

Note

This notion of entity independence can be modified by the application developer usingthe concept of cascades. Cascades allow certain operations to continue (or "cascade")across an association from one entity to another. Cascades are covered in detail inChapter 8, Association Mappings.

6.3. Significance of type categories

Why do we spend so much time categorizing the various types of types? What is the significance of the distinction?

The main categorization was between entity types and value types. To review we said that entities, by nature of theirunique identifier, exist independently of other objects whereas values do not. An application cannot "delete" a Productsku; instead, the sku is removed when the Product itself is deleted (obviously you can update the sku of that Product to nullto make it "go away", but even there the access is done through the Product).

Nor can you define an association to that Product sku. You can define an association to Product based on its sku,assuming sku is unique, but that is totally different.

TBC...

6.4. Custom types

Hibernate makes it relatively easy for developers to create their own value types. For example, you might want to persistproperties of type java.lang.BigInteger to VARCHAR columns. Custom types are not limited to mapping values to a single

table column. So, for example, you might want to concatenate together FIRST_NAME, INITIAL and SURNAME columns into a

java.lang.String.

There are 3 approaches to developing a custom Hibernate type. As a means of illustrating the different approaches, letsconsider a use case where we need to compose a java.math.BigDecimal and java.util.Currency together into a custom Money

class.


The first approach is to directly implement the org.hibernate.type.Type interface (or one of its derivatives). Probably, you will

be more interested in the more specific org.hibernate.type.BasicType contract which would allow registration of the type (see

Section 6.5, “Type registry”). The benefit of this registration is that whenever the metadata for a particular property does notspecify the Hibernate type to use, Hibernate will consult the registry for the exposed property type. In our example, theproperty type would be Money, which is the key we would use to register our type in the registry:

Example 6.1. Defining and registering the custom Type

public class MoneyType implements BasicType { public String[] getRegistrationKeys() { return new String[] { Money.class.getName() }; }

public int[] sqlTypes(Mapping mapping) { // We will simply use delegation to the standard basic types for BigDecimal and Currency for many of the // Type methods... return new int[] { BigDecimalType.INSTANCE.sqlType(), CurrencyType.INSTANCE.sqlType(), }; // we could also have honored any registry overrides via... //return new int[] { // mappings.getTypeResolver().basic( BigDecimal.class.getName() ).sqlTypes( mappings )[0], // mappings.getTypeResolver().basic( Currency.class.getName() ).sqlTypes( mappings )[0] //}; }

public Class getReturnedClass() { return Money.class; }

public Object nullSafeGet(ResultSet rs, String[] names, SessionImplementor session, Object owner) throws SQLException { assert names.length == 2; BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check return amount == null && currency == null ? null : new Money( amount, currency ); }

public void nullSafeSet(PreparedStatement st, Object value, int index, boolean[] settable, SessionImplementor session) throws SQLException { if ( value == null ) { BigDecimalType.INSTANCE.set( st, null, index ); CurrencyType.INSTANCE.set( st, null, index+1 ); } else { final Money money = (Money) value; BigDecimalType.INSTANCE.set( st, money.getAmount(), index ); CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 ); } }

...}

Configuration cfg = new Configuration();cfg.registerTypeOverride( new MoneyType() );cfg...;

Important

It is important that we registered the type before adding mappings.


Note

B o th org.hibernate.usertype.UserType and org.hibernate.usertype.CompositeUserType were

originally added to isolate user code from internal changes to the org.hibernate.type.Type

interfaces.

The second approach is the use the org.hibernate.usertype.UserType interface, which presents a somewhat simplified view

of the org.hibernate.type.Type interface. Using a org.hibernate.usertype.UserType, our Money custom type would look as

follows:

Example 6.2. Defining the custom UserType

public class MoneyType implements UserType { public int[] sqlTypes() { return new int[] { BigDecimalType.INSTANCE.sqlType(), CurrencyType.INSTANCE.sqlType(), }; }


public Object nullSafeGet(ResultSet rs, String[] names, Object owner) throws SQLException { assert names.length == 2; BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check return amount == null && currency == null ? null : new Money( amount, currency ); }

public void nullSafeSet(PreparedStatement st, Object value, int index) throws SQLException { if ( value == null ) { BigDecimalType.INSTANCE.set( st, null, index ); CurrencyType.INSTANCE.set( st, null, index+1 ); } else { final Money money = (Money) value; BigDecimalType.INSTANCE.set( st, money.getAmount(), index ); CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 ); } }

...}

There is not much difference between the org.hibernate.type.Type example and the org.hibernate.usertype.UserType example,

but that is only because of the snippets shown. If you choose the org.hibernate.type.Type approach there are quite a few

more methods you would need to implement as compared to the org.hibernate.usertype.UserType.


The third and final approach is the use the org.hibernate.usertype.CompositeUserType interface, which differs from

org.hibernate.usertype.UserType in that it gives us the ability to provide Hibernate the information to handle the composition

within the Money class (specifically the 2 attributes). This would give us the capability, for example, to reference the amount

attribute in an HQL query. Using a org.hibernate.usertype.CompositeUserType, our Money custom type would look as follows:

Example 6.3. Defining the custom CompositeUserType

public class MoneyType implements CompositeUserType { public String[] getPropertyNames() { // ORDER IS IMPORTANT! it must match the order the columns are defined in the property mapping return new String[] { "amount", "currency" }; }

public Type[] getPropertyTypes() { return new Type[] { BigDecimalType.INSTANCE, CurrencyType.INSTANCE }; }


public Object getPropertyValue(Object component, int propertyIndex) { if ( component == null ) { return null; }

final Money money = (Money) component; switch ( propertyIndex ) { case 0: { return money.getAmount(); } case 1: { return money.getCurrency(); } default: { throw new HibernateException( "Invalid property index [" + propertyIndex + "]" ); } } }

public void setPropertyValue(Object component, int propertyIndex, Object value) throws HibernateException { if ( component == null ) { return; }

final Money money = (Money) component; switch ( propertyIndex ) { case 0: { money.setAmount( (BigDecimal) value ); break; } case 1: { money.setCurrency( (Currency) value ); break; } default: { throw new HibernateException( "Invalid property index [" + propertyIndex + "]" ); } } }

public Object nullSafeGet(ResultSet rs, String[] names, SessionImplementor session, Object owner) throws SQLException { assert names.length == 2; BigDecimal amount = BigDecimalType.INSTANCE.get( names[0] ); // already handles null check Currency currency = CurrencyType.INSTANCE.get( names[1] ); // already handles null check return amount == null && currency == null ? null : new Money( amount, currency ); }

public void nullSafeSet(PreparedStatement st, Object value, int index, SessionImplementor session) throws SQLException { if ( value == null ) { BigDecimalType.INSTANCE.set( st, null, index ); CurrencyType.INSTANCE.set( st, null, index+1 ); } else { final Money money = (Money) value; BigDecimalType.INSTANCE.set( st, money.getAmount(), index ); CurrencyType.INSTANCE.set( st, money.getCurrency(), index+1 ); } }

...}

6.5. Type registry

Internally Hibernate uses a registry of basic types (see Section 6.1.1, “Basic value types”) when it needs to resolve thespecific org.hibernate.type.Type to use in certain situations. It also provides a way for applications to add extra basic type

registrations as well as override the standard basic type registrations.

To register a new type or to override an existing type registration, applications would make use of the registerTypeOverride

method of the org.hibernate.cfg.Configuration class when bootstrapping Hibernate. For example, lets say you want Hibernate

to use your custom SuperDuperStringType; during bootstrap you would call:

Example 6.4. Overriding the standard StringType

Configuration cfg = ...;cfg.registerTypeOverride( new SuperDuperStringType() );

The argument to registerTypeOverride is a org.hibernate.type.BasicType which is a specialization of the org.hibernate.type.Type

we saw before. It adds a single method:

Example 6.5. Snippet from BasicType.java

/** * Get the names under which this type should be registered in the type registry. * * @return The keys under which to register this type. */ public String[] getRegistrationKeys();

One approach is to use inheritance (SuperDuperStringType extends org.hibernate.type.StringType); another is to use

delegation.

Chapter 7. Collection mapping

Table of Contents

7.1. Persistent collections7.2. How to map collections

7.2.1. Collection foreign keys7.2.2. Indexed collections7.2.3. Collections of basic types and embeddable objects


7.3.1. Sorted collections7.3.2. Bidirectional associations7.3.3. Bidirectional associations with indexed collections7.3.4. Ternary associations7.3.5. Using an <idbag>


7.1. Persistent collections

Naturally Hibernate also allows to persist collections. These persistent collections can contain almost any other Hibernatetype, including: basic types, custom types, components and references to other entities. The distinction between value andreference semantics is in this context very important. An object in a collection might be handled with "value" semantics (itslife cycle fully depends on the collection owner), or it might be a reference to another entity with its own life cycle. In thelatter case, only the "link" between the two objects is considered to be a state held by the collection.

As a requirement persistent collection-valued fields must be declared as an interface type (see Example 7.2, “Collectionmapping using @OneToMany and @JoinColumn”). The actual interface might be java.util.Set, java.util.Collection, java.util.List,

java.util.Map, java.util.SortedSet, java.util.SortedMap or anything you like ("anything you like" means you will have to write an

implementation of org.hibernate.usertype.UserCollectionType).

Notice how in Example 7.2, “Collection mapping using @OneToMany and @JoinColumn” the instance variable parts was

initialized with an instance of HashSet. This is the best way to initialize collection valued properties of newly instantiated

(non-persistent) instances. When you make the instance persistent, by calling persist(), Hibernate will actually replace the

HashSet with an instance of Hibernate's own implementation of Set. Be aware of the following error:

Example 7.1. Hibernate uses its own collection implementations

Cat cat = new DomesticCat();Cat kitten = new DomesticCat();....Set kittens = new HashSet();kittens.add(kitten);cat.setKittens(kittens);session.persist(cat);

kittens = cat.getKittens(); // Okay, kittens collection is a Set(HashSet) cat.getKittens(); // Error!

The persistent collections injected by Hibernate behave like HashMap, HashSet, TreeMap, TreeSet or ArrayList, depending on

the interface type.

Collections instances have the usual behavior of value types. They are automatically persisted when referenced by apersistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object toanother, its elements might be moved from one table to another. Two entities cannot share a reference to the samecollection instance. Due to the underlying relational model, collection-valued properties do not support null valuesemantics. Hibernate does not distinguish between a null collection reference and an empty collection.

Note

Use persistent collections the same way you use ordinary Java collections. However,ensure you understand the semantics of bidirectional associations (see Section 7.3.2,“Bidirectional associations”).

7.2. How to map collections

Using annotations you can map Collections , Lists , Maps and Sets of associated entities using @OneToMany and

@ManyToMany. For collections of a basic or embeddable type use @ElementCollection. In the simplest case a collectionmapping looks like this:

Example 7.2. Collection mapping using @OneToMany and @JoinColumn

@Entitypublic class Product {

private String serialNumber; private Set<Part> parts = new HashSet<Part>();

@Id public String getSerialNumber() { return serialNumber; } void setSerialNumber(String sn) { serialNumber = sn; } @OneToMany @JoinColumn(name="PART_ID") public Set<Part> getParts() { return parts; } void setParts(Set parts) { this.parts = parts; }}

@Entitypublic class Part { ...}

Product describes a unidirectional relationship with Part using the join column PART_ID. In this unidirectional one to manyscenario you can also use a join table as seen in Example 7.3, “Collection mapping using @OneToMany and@JoinTable”.

Example 7.3. Collection mapping using @OneToMany and @JoinTable

@Entitypublic class Product {

private String serialNumber; private Set<Part> parts = new HashSet<Part>();

@Id public String getSerialNumber() { return serialNumber; } void setSerialNumber(String sn) { serialNumber = sn; } @OneToMany @JoinTable( name="PRODUCT_PARTS", joinColumns = @JoinColumn( name="PRODUCT_ID"), inverseJoinColumns = @JoinColumn( name="PART_ID") ) public Set<Part> getParts() { return parts; } void setParts(Set parts) { this.parts = parts; }}

@Entitypublic class Part { ...}

Without describing any physical mapping (no @JoinColumn or @JoinTable), a unidirectional one to many with join table is

used. The table name is the concatenation of the owner table name, _, and the other side table name. The foreign keyname(s) referencing the owner table is the concatenation of the owner table, _, and the owner primary key column(s)name. The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and theother side primary key column(s) name. A unique constraint is added to the foreign key referencing the other side table toreflect the one to many.

Lets have a look now how collections are mapped using Hibernate mapping files. In this case the first step is to chose theright mapping element. It depends on the type of interface. For example, a <set> element is used for mapping properties of

type Set.

Example 7.4. Mapping a Set using <set>

<class name="Product"> <id name="serialNumber" column="productSerialNumber"/> <set name="parts"> <key column="productSerialNumber" not-null="true"/> <one-to-many class="Part"/> </set></class>

In Example 7.4, “Mapping a Set using <set>” a one-to-many association links the Product and Part entities. This association

requires the existence of a foreign key column and possibly an index column to the Part table. This mapping loses certain

semantics of normal Java collections:

An instance of the contained entity class cannot belong to more than one instance of the collection.

An instance of the contained entity class cannot appear at more than one value of the collection index.

Looking closer at the used <one-to-many> tag we see that it has the following options.

Example 7.5. options of <one-to-many> element

<one-to-many class="ClassName" not-found="ignore|exception" entity-name="EntityName" node="element-name" embed-xml="true|false" />

class (required): the name of the associated class.

not-found (optional - defaults to exception): specifies how cached identifiers that reference missing rows will be

handled. ignore will treat a missing row as a null association.

entity-name (optional): the entity name of the associated class, as an alternative to class.

The <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the table name anywhere.

Warning

If the foreign key column of a <one-to-many> association is declared NOT NULL, you must

declare the <key> mapping not-null="true" or use a bidirectional association with the

collection mapping marked inverse="true". See Section 7.3.2, “Bidirectional associations”.

Apart from the <set> tag as shown in Example 7.4, “Mapping a Set using <set>”, there is also <list>, <map>, <bag>, <array>

and <primitive-array> mapping elements. The <map> element is representative:

Example 7.6. Elements of the <map> mapping

<map name="propertyName" table="table_name" schema="schema_name" lazy="true|extra|false" inverse="true|false" cascade="all|none|save-update|delete|all-delete-orphan|delete-orphan" sort="unsorted|natural|comparatorClass" order-by="column_name asc|desc" where="arbitrary sql where condition" fetch="join|select|subselect" batch-size="N" access="field|property|ClassName" optimistic-lock="true|false" mutable="true|false" node="element-name|." embed-xml="true|false">

<key .... /> <map-key .... /> <element .... /></map>

name: the collection property name

table (optional - defaults to property name): the name of the collection table. It is not used for one-to-many

associations.

schema (optional): the name of a table schema to override the schema declared on the root element

lazy (optional - defaults to true): disables lazy fetching and specifies that the association is always eagerly

fetched. It can also be used to enable "extra-lazy" fetching where most operations do not initialize the collection.This is suitable for large collections.

inverse (optional - defaults to false): marks this collection as the "inverse" end of a bidirectional association.

cascade (optional - defaults to none): enables operations to cascade to child entities.

sort (optional): specifies a sorted collection with natural sort order or a given comparator class.

order-by (optional): specifies a table column or columns that define the iteration order of the Map, Set or bag,

together with an optional asc or desc.

where (optional): specifies an arbitrary SQL WHERE condition that is used when retrieving or removing the

collection. This is useful if the collection needs to contain only a subset of the available data.

fetch (optional, defaults to select): chooses between outer-join fetching, fetching by sequential select, and

fetching by sequential subselect.

batch-size (optional, defaults to 1): specifies a "batch size" for lazily fetching instances of this collection.

access (optional - defaults to property): the strategy Hibernate uses for accessing the collection property value.

optimistic-lock (optional - defaults to true): specifies that changes to the state of the collection results in increments

of the owning entity's version. For one-to-many associations you may want to disable this setting.

mutable (optional - defaults to true): a value of false specifies that the elements of the collection never change.

This allows for minor performance optimization in some cases.

After exploring the basic mapping of collections in the preceding paragraphs we will now focus details like physicalmapping considerations, indexed collections and collections of value types.

7.2.1. Collection foreign keys

On the database level collection instances are distinguished by the foreign key of the entity that owns the collection. Thisforeign key is referred to as the collection key column, or columns, of the collection table. The collection key column ismapped by the @JoinColumn annotation respectively the <key> XML element.

There can be a nullability constraint on the foreign key column. For most collections, this is implied. For unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need to specify

@JoinColumn(nullable=false)

or

<key column="productSerialNumber" not-null="true"/>

The foreign key constraint can use ON DELETE CASCADE. In XML this can be expressed via:

<key column="productSerialNumber" on-delete="cascade"/>

In annotations the Hibernate specific annotation @OnDelete has to be used.

@OnDelete(action=OnDeleteAction.CASCADE)

See Section 5.1.11.3, “Key” for more information about the <key> element.

7.2.2. Indexed collections

In the following paragraphs we have a closer look at the indexed collections List and Map how the their index can be

mapped in Hibernate.

7.2.2.1. Lists

Lists can be mapped in two different ways:

as ordered lists, where the order is not materialized in the database

as indexed lists, where the order is materialized in the database

To order lists in memory, add @javax.persistence.OrderBy to your property. This annotation takes as parameter a list of

comma separated properties (of the target entity) and orders the collection accordingly (egfirstname asc, age desc, weight asc nulls last), if the string is empty, the collection will be ordered by the primary key of the

target entity.

Example 7.7. Ordered lists using @OrderBy

@Entitypublic class Customer { @Id @GeneratedValue public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } private Integer id;

@OneToMany(mappedBy="customer") @OrderBy("number") public List<Order> getOrders() { return orders; } public void setOrders(List<Order> orders) { this.orders = orders; } private List<Order> orders;}

@Entitypublic class Order { @Id @GeneratedValue public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } private Integer id;

public String getNumber() { return number; } public void setNumber(String number) { this.number = number; } private String number;

@ManyToOne public Customer getCustomer() { return customer; } public void setCustomer(Customer customer) { this.customer = customer; } private Customer number;}

-- Table schema|-------------| |----------|| Order | | Customer ||-------------| |----------|| id | | id || number | |----------| | customer_id ||-------------|

To store the index value in a dedicated column, use the @javax.persistence.OrderColumn annotation on your property. This

annotations describes the column name and attributes of the column keeping the index value. This column is hosted onthe table containing the association foreign key. If the column name is not specified, the default is the name of thereferencing property, followed by underscore, followed by ORDER (in the following example, it would be orders_ORDER).

Example 7.8. Explicit index column using @OrderColumn


@OneToMany(mappedBy="customer") @OrderColumn(name="orders_index") public List<Order> getOrders() { return orders; } public void setOrders(List<Order> orders) { this.orders = orders; } private List<Order> orders;}




-- Table schema|--------------| |----------|| Order | | Customer ||--------------| |----------|| id | | id || number | |----------| | customer_id || orders_order ||--------------|

Note

We recommend you to convert the legacy @org.hibernate.annotations.IndexColumn usages

to the JPA standard @javax.persistence.OrderColumn.

If you are leveraging a custom list index base (maybe currently using theorg.hibernate.annotations.IndexColumn.literal attribute), you can specify this using the

@org.hibernate.annotations.ListIndexBase in conjunction with @javax.persistence.OrderColumn.

The default base is 0 like in Java.

Looking again at the Hibernate mapping file equivalent, the index of an array or list is always of type integer and is mapped

using the <list-index> element. The mapped column contains sequential integers that are numbered from zero by default.

Example 7.9. index-list element for indexed collections in xml mapping

<list-index column="column_name" base="0|1|..."/>

column_name (required): the name of the column holding the collection index values.

base (optional - defaults to 0): the value of the index column that corresponds to the first element of the list or

array.

If your table does not have an index column, and you still wish to use List as the property type, you can map the property

as a Hibernate <bag>. A bag does not retain its order when it is retrieved from the database, but it can be optionally sortedor ordered.

7.2.2.2. Maps

The question with Maps is where the key value is stored. There are everal options. Maps can borrow their keys from one of

the associated entity properties or have dedicated columns to store an explicit key.

To use one of the target entity property as a key of the map, use @MapKey(name="myProperty"), where myProperty is a

property name in the target entity. When using @MapKey without the name attribuate, the target entity primary key is used.

The map key uses the same column as the property pointed out. There is no additional column defined to hold the mapkey, because the map key represent a target property. Be aware that once loaded, the key is no longer kept in sync withthe property. In other words, if you change the property value, the key will not change automatically in your Java model.

Example 7.10. Use of target entity property as map key via @MapKey


@OneToMany(mappedBy="customer") @MapKey(name="number") public Map<String,Order> getOrders() { return orders; } public void setOrders(Map<String,Order> order) { this.orders = orders; } private Map<String,Order> orders;}




-- Table schema|-------------| |----------|| Order | | Customer ||-------------| |----------|| id | | id || number | |----------| | customer_id ||-------------|

Alternatively the map key is mapped to a dedicated column or columns. In order to customize the mapping use one of thefollowing annotations:

@MapKeyColumn if the map key is a basic type. If you don't specify the column name, the name of the property followed

by underscore followed by KEY is used (for example orders_KEY).

@MapKeyEnumerated / @MapKeyTemporal if the map key type is respectively an enum or a Date.

@MapKeyJoinColumn/@MapKeyJoinColumns if the map key type is another entity.

@AttributeOverride/@AttributeOverrides when the map key is a embeddable object. Use key. as a prefix for your

embeddable object property names.

You can also use @MapKeyClass to define the type of the key if you don't use generics.

Example 7.11. Map key as basic type using @MapKeyColumn


@OneToMany @JoinTable(name="Cust_Order") @MapKeyColumn(name="orders_number") public Map<String,Order> getOrders() { return orders; } public void setOrders(Map<String,Order> orders) { this.orders = orders; } private Map<String,Order> orders;}




-- Table schema|-------------| |----------| |---------------|| Order | | Customer | | Cust_Order ||-------------| |----------| |---------------|| id | | id | | customer_id || number | |----------| | order_id || customer_id | | orders_number ||-------------| |---------------|

Note

We recommend you to migrate from @org.hibernate.annotations.MapKey /

@org.hibernate.annotation.MapKeyManyToMany to the new standard approach described

above

Using Hibernate mapping files there exists equivalent concepts to the descibed annotations. You have to use <map-key>,

<map-key-many-to-many> and <composite-map-key>. <map-key> is used for any basic type, <map-key-many-to-many> for an

entity reference and <composite-map-key> for a composite type.

Example 7.12. map-key xml mapping element

<map-key column="column_name" formula="any SQL expression" type="type_name" node="@attribute-name" length="N"/>

column (optional): the name of the column holding the collection index values.

formula (optional): a SQL formula used to evaluate the key of the map.

type (required): the type of the map keys.

Example 7.13. map-key-many-to-many

<map-key-many-to-many column="column_name" formula="any SQL expression" class="ClassName"/>

column (optional): the name of the foreign key column for the collection index values.

formula (optional): a SQ formula used to evaluate the foreign key of the map key.

class (required): the entity class used as the map key.

7.2.3. Collections of basic types and embeddable objects

In some situations you don't need to associate two entities but simply create a collection of basic types or embeddableobjects. Use the @ElementCollection for this case.

Example 7.14. Collection of basic types mapped via @ElementCollection

@Entitypublic class User { [...] public String getLastname() { ...}

@ElementCollection @CollectionTable(name="Nicknames", joinColumns=@JoinColumn(name="user_id")) @Column(name="nickname") public Set<String> getNicknames() { ... } }

The collection table holding the collection data is set using the @CollectionTable annotation. If omitted the collection table

name defaults to the concatenation of the name of the containing entity and the name of the collection attribute, separatedby an underscore. In our example, it would be User_nicknames.

The column holding the basic type is set using the @Column annotation. If omitted, the column name defaults to the

property name: in our example, it would be nicknames.

But you are not limited to basic types, the collection type can be any embeddable object. To override the columns of theembeddable object in the collection table, use the @AttributeOverride annotation.

Example 7.15. @ElementCollection for embeddable objects

@Entitypublic class User { [...] public String getLastname() { ...}

@ElementCollection @CollectionTable(name="Addresses", joinColumns=@JoinColumn(name="user_id")) @AttributeOverrides({ @AttributeOverride(name="street1", column=@Column(name="fld_street")) }) public Set<Address> getAddresses() { ... } }

@Embeddablepublic class Address { public String getStreet1() {...} [...]}

Such an embeddable object cannot contains a collection itself.

Note

i n @AttributeOverride, you must use the value. prefix to override properties of the

embeddable object used in the map value and the key. prefix to override properties of the

embeddable object used in the map key.

@Entitypublic class User { @ElementCollection @AttributeOverrides({ @AttributeOverride(name="key.street1", column=@Column(name="fld_street")), @AttributeOverride(name="value.stars", column=@Column(name="fld_note")) }) public Map<Address,Rating> getFavHomes() { ... }

Note

We recommend you to migrate from @org.hibernate.annotations.CollectionOfElements to the

new @ElementCollection annotation.

Using the mapping file approach a collection of values is mapped using the <element> tag. For example:

Example 7.16. <element> tag for collection values using mapping files

<element column="column_name" formula="any SQL expression" type="typename" length="L" precision="P" scale="S" not-null="true|false" unique="true|false" node="element-name"/>

column (optional): the name of the column holding the collection element values.

formula (optional): an SQL formula used to evaluate the element.

type (required): the type of the collection element.


7.3.1. Sorted collections

Hibernate supports collections implementing java.util.SortedMap and java.util.SortedSet. With annotations you declare a sort

comparator using @Sort. You chose between the comparator types unsorted, natural or custom. If you want to use your

own comparator implementation, you'll also have to specify the implementation class using the comparator attribute. Note

that you need to use either a SortedSet or a SortedMap interface.

Example 7.17. Sorted collection with @Sort

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)@JoinColumn(name="CUST_ID")@Sort(type = SortType.COMPARATOR, comparator = TicketComparator.class)public SortedSet<Ticket> getTickets() { return tickets;}

Using Hibernate mapping files you specify a comparator in the mapping file with <sort>:

Example 7.18. Sorted collection using xml mapping

<set name="aliases" table="person_aliases" sort="natural"> <key column="person"/> <element column="name" type="string"/></set>

<map name="holidays" sort="my.custom.HolidayComparator"> <key column="year_id"/> <map-key column="hol_name" type="string"/> <element column="hol_date" type="date"/></map>

Allowed values of the sort attribute are unsorted, natural and the name of a class implementing java.util.Comparator.

Tip

Sorted collections actually behave like java.util.TreeSet or java.util.TreeMap.

If you want the database itself to order the collection elements, use the order-by attribute of set, bag or map mappings. This

solution is implemented using LinkedHashSet or LinkedHashMap and performs the ordering in the SQL query and not in the

memory.

Example 7.19. Sorting in database using order-by

<set name="aliases" table="person_aliases" order-by="lower(name) asc"> <key column="person"/> <element column="name" type="string"/></set>

<map name="holidays" order-by="hol_date, hol_name"> <key column="year_id"/> <map-key column="hol_name" type="string"/> <element column="hol_date type="date"/></map>

Note

The value of the order-by attribute is an SQL ordering, not an HQL ordering.

Associations can even be sorted by arbitrary criteria at runtime using a collection filter():

Example 7.20. Sorting via a query filter

sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();

7.3.2. Bidirectional associations

A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional associationare supported:

one-to-many

set or bag valued at one end and single-valued at the other

many-to-many

set or bag valued at both ends

Often there exists a many to one association which is the owner side of a bidirectional relationship. The correspondingone to many association is in this case annotated by @OneToMany(mappedBy=...)

Example 7.21. Bidirectional one to many with many to one side as association owner

@Entitypublic class Troop { @OneToMany(mappedBy="troop") public Set<Soldier> getSoldiers() { ...}

@Entitypublic class Soldier { @ManyToOne @JoinColumn(name="troop_fk") public Troop getTroop() { ...}

Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to (must not)

define any physical mapping in the mappedBy side.

To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the mappedBy

element and set the many to one @JoinColumn as insertable and updatable to false. This solution is not optimized and will

produce additional UPDATE statements.

Example 7.22. Bidirectional association with one to many side as owner

@Entitypublic class Troop { @OneToMany @JoinColumn(name="troop_fk") //we need to duplicate the physical information public Set<Soldier> getSoldiers() { ...}

@Entitypublic class Soldier { @ManyToOne @JoinColumn(name="troop_fk", insertable=false, updatable=false) public Troop getTroop() { ...}

How does the mappping of a bidirectional mapping look like in Hibernate mapping xml? There you define a bidirectionalone-to-many association by mapping a one-to-many association to the same table column(s) as a many-to-one

association and declaring the many-valued end inverse="true".

Example 7.23. Bidirectional one to many via Hibernate mapping files

<class name="Parent"> <id name="id" column="parent_id"/> .... <set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/> </set></class>

<class name="Child"> <id name="id" column="child_id"/> .... <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/></class>

Mapping one end of an association with inverse="true" does not affect the operation of cascades as these are orthogonal

concepts.

A many-to-many association is defined logically using the @ManyToMany annotation. You also have to describe the

association table and the join conditions using the @JoinTable annotation. If the association is bidirectional, one side has

to be the owner and one side has to be the inverse end (ie. it will be ignored when updating the relationship values in theassociation table):

Example 7.24. Many to many association via @ManyToMany

@Entitypublic class Employer implements Serializable { @ManyToMany( targetEntity=org.hibernate.test.metadata.manytomany.Employee.class, cascade={CascadeType.PERSIST, CascadeType.MERGE} ) @JoinTable( name="EMPLOYER_EMPLOYEE", joinColumns=@JoinColumn(name="EMPER_ID"), inverseJoinColumns=@JoinColumn(name="EMPEE_ID") ) public Collection getEmployees() { return employees; } ...}

@Entitypublic class Employee implements Serializable { @ManyToMany( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, mappedBy = "employees", targetEntity = Employer.class ) public Collection getEmployers() { return employers; }}

In this example @JoinTable defines a name, an array of join columns, and an array of inverse join columns. The latter ones

are the columns of the association table which refer to the Employee primary key (the "other side"). As seen previously, the

other side don't have to (must not) describe the physical mapping: a simple mappedBy argument containing the owner side

property name bind the two.

As any other annotations, most values are guessed in a many to many relationship. Without describing any physicalmapping in a unidirectional many to many the following rules applied. The table name is the concatenation of the ownertable name, _ and the other side table name. The foreign key name(s) referencing the owner table is the concatenation

of the owner table name, _ and the owner primary key column(s). The foreign key name(s) referencing the other side is

the concatenation of the owner property name, _ , and the other side primary key column(s). These are the same rules

used for a unidirectional one to many relationship.

Example 7.25. Default values for @ManyToMany (uni-directional)

@Entitypublic class Store { @ManyToMany(cascade = CascadeType.PERSIST) public Set<City> getImplantedIn() { ... }}

@Entitypublic class City { ... //no bidirectional relationship}

A Store_City is used as the join table. The Store_id column is a foreign key to the Store table. The implantedIn_id column is a

foreign key to the City table.

Without describing any physical mapping in a bidirectional many to many the following rules applied. The table name isthe concatenation of the owner table name, _ and the other side table name. The foreign key name(s) referencing the

owner table is the concatenation of the other side property name, _ , and the owner primary key column(s). The foreign

key name(s) referencing the other side is the concatenation of the owner property name, _ , and the other side primary

key column(s). These are the same rules used for a unidirectional one to many relationship.

Example 7.26. Default values for @ManyToMany (bi-directional)

@Entitypublic class Store { @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}) public Set<Customer> getCustomers() { ... }}

@Entitypublic class Customer { @ManyToMany(mappedBy="customers") public Set<Store> getStores() { ... }}

A Store_Customer is used as the join table. The stores_id column is a foreign key to the Store table. The customers_id column

is a foreign key to the Customer table.

Using Hibernate mapping files you can map a bidirectional many-to-many association by mapping two many-to-manyassociations to the same database table and declaring one end as inverse.

Note

You cannot select an indexed collection.

Example 7.27, “Many to many association using Hibernate mapping files” shows a bidirectional many-to-many associationthat illustrates how each category can have many items and each item can be in many categories:

Example 7.27. Many to many association using Hibernate mapping files

<class name="Category"> <id name="id" column="CATEGORY_ID"/> ... <bag name="items" table="CATEGORY_ITEM"> <key column="CATEGORY_ID"/> <many-to-many class="Item" column="ITEM_ID"/> </bag></class>

<class name="Item"> <id name="id" column="ITEM_ID"/> ...

 <bag name="categories" table="CATEGORY_ITEM" inverse="true"> <key column="ITEM_ID"/> <many-to-many class="Category" column="CATEGORY_ID"/> </bag></class>

Changes made only to the inverse end of the association are not persisted. This means that Hibernate has tworepresentations in memory for every bidirectional association: one link from A to B and another link from B to A. This iseasier to understand if you think about the Java object model and how a many-to-many relationship in Javais created:

Example 7.28. Effect of inverse vs. non-inverse side of many to many associations

category.getItems().add(item); // The category now "knows" about the relationshipitem.getCategories().add(category); // The item now "knows" about the relationship

session.persist(item); // The relationship won't be saved!session.persist(category); // The relationship will be saved

The non-inverse side is used to save the in-memory representation to the database.

7.3.3. Bidirectional associations with indexed collections

There are some additional considerations for bidirectional mappings with indexed collections (where one end isrepresented as a <list> or <map>) when using Hibernate mapping files. If there is a property of the child class that maps to

the index column you can use inverse="true" on the collection mapping:

Example 7.29. Bidirectional association with indexed collection

<class name="Parent"> <id name="id" column="parent_id"/> .... <map name="children" inverse="true"> <key column="parent_id"/> <map-key column="name" type="string"/> <one-to-many class="Child"/> </map></class>

<class name="Child"> <id name="id" column="child_id"/> .... <property name="name" not-null="true"/> <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/></class>

If there is no such property on the child class, the association cannot be considered truly bidirectional. That is, there isinformation available at one end of the association that is not available at the other end. In this case, you cannot map thecollection inverse="true". Instead, you could use the following mapping:

Example 7.30. Bidirectional association with indexed collection, but no index column

<class name="Parent"> <id name="id" column="parent_id"/> .... <map name="children"> <key column="parent_id" not-null="true"/> <map-key column="name" type="string"/> <one-to-many class="Child"/> </map></class>

<class name="Child"> <id name="id" column="child_id"/> .... <many-to-one name="parent" class="Parent" column="parent_id" insert="false" update="false" not-null="true"/></class>

Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key.

7.3.4. Ternary associations

There are three possible approaches to mapping a ternary association. One approach is to use a Map with an association

as its index:

Example 7.31. Ternary association mapping

@Entitypublic class Company { @Id int id; ... @OneToMany // unidirectional @MapKeyJoinColumn(name="employee_id") Map<Employee, Contract> contracts;}

// or

<map name="contracts"> <key column="employer_id" not-null="true"/> <map-key-many-to-many column="employee_id" class="Employee"/> <one-to-many class="Contract"/></map>

A second approach is to remodel the association as an entity class. This is the most common approach. A final alternativeis to use composite elements, which will be discussed later.

7.3.5. Using an <idbag>

The majority of the many-to-many associations and collections of values shown previously all map to tables withcomposite keys, even though it has been suggested that entities should have synthetic identifiers (surrogate keys). A pureassociation table does not seem to benefit much from a surrogate key, although a collection of composite values might.For this reason Hibernate provides a feature that allows you to map many-to-many associations and collections of valuesto a table with a surrogate key.

The <idbag> element lets you map a List (or Collection) with bag semantics. For example:

<idbag name="lovers" table="LOVERS"> <collection-id column="ID" type="long"> <generator class="sequence"/> </collection-id> <key column="PERSON1"/> <many-to-many column="PERSON2" class="Person" fetch="join"/></idbag>

An <idbag> has a synthetic id generator, just like an entity class. A different surrogate key is assigned to each collection

row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value of a particular row.

The update performance of an <idbag> supersedes a regular <bag>. Hibernate can locate individual rows efficiently and

update or delete them individually, similar to a list, map or set.

In the current implementation, the native identifier generation strategy is not supported for <idbag> collection identifiers.


This section covers collection examples.

The following class has a collection of Child instances:

Example 7.32. Example classes Parent and Child

public class Parent { private long id; private Set<Child> children;

// getter/setter ...}

public class Child { private long id; private String name


If each child has, at most, one parent, the most natural mapping is a one-to-many association:

Example 7.33. One to many unidirectional Parent-Child relationship using annotations

public class Parent { @Id @GeneratedValue private long id;

@OneToMany private Set<Child> children;


public class Child { @Id @GeneratedValue private long id; private String name;


Example 7.34. One to many unidirectional Parent-Child relationship using mapping files

<hibernate-mapping>

<class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class>

<class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class>


This maps to the following table definitions:

Example 7.35. Table definitions for unidirectional Parent-Child relationship

create table parent ( id bigint not null primary key )create table child ( id bigint not null primary key, name varchar(255), parent_id bigint )alter table child add constraint childfk0 (parent_id) references parent

If the parent is required, use a bidirectional one-to-many association:

Example 7.36. One to many bidirectional Parent-Child relationship using annotations


@OneToMany(mappedBy="parent") private Set<Child> children;


public class Child { @Id @GeneratedValue private long id;

private String name; @ManyToOne private Parent parent;


Example 7.37. One to many bidirectional Parent-Child relationship using mapping files

<hibernate-mapping>

<class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class>

<class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/> </class>


Notice the NOT NULL constraint:

Example 7.38. Table definitions for bidirectional Parent-Child relationship

create table parent ( id bigint not null primary key )create table child ( id bigint not null primary key, name varchar(255), parent_id bigint not null )alter table child add constraint childfk0 (parent_id) references parent

Alternatively, if this association must be unidirectional you can enforce the NOT NULL constraint.

Example 7.39. Enforcing NOT NULL constraint in unidirectional relation using annotations


@OneToMany(optional=false) private Set<Child> children;


public class Child { @Id @GeneratedValue private long id; private String name;


Example 7.40. Enforcing NOT NULL constraint in unidirectional relation using mapping files

<hibernate-mapping>

<class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id" not-null="true"/> <one-to-many class="Child"/> </set> </class>



On the other hand, if a child has multiple parents, a many-to-many association is appropriate.

Example 7.41. Many to many Parent-Child relationship using annotations


@ManyToMany private Set<Child> children;


public class Child { @Id @GeneratedValue private long id;

private String name;


Example 7.42. Many to many Parent-Child relationship using mapping files

<hibernate-mapping>

<class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" table="childset"> <key column="parent_id"/> <many-to-many class="Child" column="child_id"/> </set> </class>



Table definitions:

Example 7.43. Table definitions for many to many releationship

create table parent ( id bigint not null primary key )create table child ( id bigint not null primary key, name varchar(255) )create table childset ( parent_id bigint not null, child_id bigint not null, primary key ( parent_id, child_id ) )alter table childset add constraint childsetfk0 (parent_id) references parentalter table childset add constraint childsetfk1 (child_id) references child

For more examples and a complete explanation of a parent/child relationship mapping, see Chapter 23, Example:Parent/Child for more information. Even more complex association mappings are covered in the next chapter.

Chapter 8. Association Mappings

Table of Contents

8.1. Introduction8.2. Unidirectional associations

8.2.1. Many-to-one8.2.2. One-to-one8.2.3. One-to-many


8.3.1. One-to-many8.3.2. Many-to-one8.3.3. One-to-one8.3.4. Many-to-many


8.4.1. one-to-many / many-to-one8.4.2. One-to-one


8.5.1. one-to-many / many-to-one8.5.2. one to one8.5.3. Many-to-many


8.1. Introduction

Association mappings are often the most difficult thing to implement correctly. In this section we examine some canonicalcases one by one, starting with unidirectional mappings and then bidirectional cases. We will use Person and Address in all

the examples.

Associations will be classified by multiplicity and whether or not they map to an intervening join table.

Nullable foreign keys are not considered to be good practice in traditional data modelling, so our examples do not usenullable foreign keys. This is not a requirement of Hibernate, and the mappings will work if you drop the nullabilityconstraints.

8.2. Unidirectional associations

8.2.1. Many-to-one

A unidirectional many-to-one association is the most common kind of unidirectional association.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" not-null="true"/></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id></class>

create table Person ( personId bigint not null primary key, addressId bigint not null )create table Address ( addressId bigint not null primary key )

8.2.2. One-to-one

A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the column uniqueconstraint.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" unique="true" not-null="true"/></class>


create table Person ( personId bigint not null primary key, addressId bigint not null unique )create table Address ( addressId bigint not null primary key )

A unidirectional one-to-one association on a primary key usually uses a special id generator In this example, however, wehave reversed the direction of the association:

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id></class>

<class name="Address"> <id name="id" column="personId"> <generator class="foreign"> <param name="property">person</param> </generator> </id> <one-to-one name="person" constrained="true"/></class>

create table Person ( personId bigint not null primary key )create table Address ( personId bigint not null primary key )

8.2.3. One-to-many

A unidirectional one-to-many association on a foreign key is an unusual case, and is not recommended.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses"> <key column="personId" not-null="true"/> <one-to-many class="Address"/> </set></class>


create table Person ( personId bigint not null primary key )create table Address ( addressId bigint not null primary key, personId bigint not null )

You should instead use a join table for this kind of association.


8.3.1. One-to-many

A unidirectional one-to-many association on a join table is the preferred option. Specifying unique="true", changes the

multiplicity from many-to-many to one-to-many.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" unique="true" class="Address"/> </set></class>


create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null, addressId bigint not null primary key )create table Address ( addressId bigint not null primary key )

8.3.2. Many-to-one

A unidirectional many-to-one association on a join table is common when the association is optional. For example:

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true"/> </join></class>


create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null primary key, addressId bigint not null )create table Address ( addressId bigint not null primary key )

8.3.3. One-to-one

A unidirectional one-to-one association on a join table is possible, but extremely unusual.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true" unique="true"/> </join></class>


create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )create table Address ( addressId bigint not null primary key )

8.3.4. Many-to-many

Finally, here is an example of a unidirectional many-to-many association.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" class="Address"/> </set></class>


create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )create table Address ( addressId bigint not null primary key )


8.4.1. one-to-many / many-to-one

A bidirectional many-to-one association is the most common kind of association. The following example illustrates thestandard parent/child relationship.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" not-null="true"/></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <set name="people" inverse="true"> <key column="addressId"/> <one-to-many class="Person"/> </set></class>

create table Person ( personId bigint not null primary key, addressId bigint not null )create table Address ( addressId bigint not null primary key )

If you use a List, or other indexed collection, set the key column of the foreign key to not null. Hibernate will manage the

association from the collections side to maintain the index of each element, making the other side virtually inverse bysetting update="false" and insert="false":

<class name="Person"> <id name="id"/> ... <many-to-one name="address" column="addressId" not-null="true" insert="false" update="false"/></class>

<class name="Address"> <id name="id"/> ... <list name="people"> <key column="addressId" not-null="true"/> <list-index column="peopleIdx"/> <one-to-many class="Person"/> </list></class>

If the underlying foreign key column is NOT NULL, it is important that you define not-null="true" on the <key> element of the

collection mapping. Do not only declare not-null="true" on a possible nested <column> element, but on the <key> element.

8.4.2. One-to-one

A bidirectional one-to-one association on a foreign key is common:

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" unique="true" not-null="true"/></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <one-to-one name="person" property-ref="address"/></class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )create table Address ( addressId bigint not null primary key )

A bidirectional one-to-one association on a primary key uses the special id generator:

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <one-to-one name="address"/></class>

<class name="Address"> <id name="id" column="personId"> <generator class="foreign"> <param name="property">person</param> </generator> </id> <one-to-one name="person" constrained="true"/></class>

create table Person ( personId bigint not null primary key )create table Address ( personId bigint not null primary key )


8.5.1. one-to-many / many-to-one

The following is an example of a bidirectional one-to-many association on a join table. The inverse="true" can go on either

end of the association, on the collection, or on the join.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" unique="true" class="Address"/> </set></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <join table="PersonAddress" inverse="true" optional="true"> <key column="addressId"/> <many-to-one name="person" column="personId" not-null="true"/> </join></class>

create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null, addressId bigint not null primary key )create table Address ( addressId bigint not null primary key )

8.5.2. one to one

A bidirectional one-to-one association on a join table is possible, but extremely unusual.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true" unique="true"/> </join></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true" inverse="true"> <key column="addressId" unique="true"/> <many-to-one name="person" column="personId" not-null="true" unique="true"/> </join></class>

create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )create table Address ( addressId bigint not null primary key )

8.5.3. Many-to-many

Here is an example of a bidirectional many-to-many association.

<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" class="Address"/> </set></class>

<class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <set name="people" inverse="true" table="PersonAddress"> <key column="addressId"/> <many-to-many column="personId" class="Person"/> </set></class>

create table Person ( personId bigint not null primary key )create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )create table Address ( addressId bigint not null primary key )


More complex association joins are extremely rare. Hibernate handles more complex situations by using SQL fragmentsembedded in the mapping document. For example, if a table with historical account information data definesaccountNumber, effectiveEndDate and effectiveStartDatecolumns, it would be mapped as follows:

<properties name="currentAccountKey"> <property name="accountNumber" type="string" not-null="true"/> <property name="currentAccount" type="boolean"> <formula>case when effectiveEndDate is null then 1 else 0 end</formula> </property></properties><property name="effectiveEndDate" type="date"/><property name="effectiveStateDate" type="date" not-null="true"/>

You can then map an association to the current instance, the one with null effectiveEndDate, by using:

<many-to-one name="currentAccountInfo" property-ref="currentAccountKey" class="AccountInfo"> <column name="accountNumber"/> <formula>'1'</formula></many-to-one>

In a more complex example, imagine that the association between Employee and Organization is maintained in an

Employment table full of historical employment data. An association to the employee's most recent employer, the one with

the most recent startDate, could be mapped in the following way:

<join> <key column="employeeId"/> <subselect> select employeeId, orgId from Employments group by orgId having startDate = max(startDate) </subselect> <many-to-one name="mostRecentEmployer" class="Organization" column="orgId"/></join>

This functionality allows a degree of creativity and flexibility, but it is more practical to handle these kinds of cases usingHQL or a criteria query.

Chapter 9. Component Mapping

Table of Contents

9.1. Dependent objects9.2. Collections of dependent objects9.3. Components as Map indices9.4. Components as composite identifiers9.5. Dynamic components

The notion of a component is re-used in several different contexts and purposes throughout Hibernate.

9.1. Dependent objects

A component is a contained object that is persisted as a value type and not an entity reference. The term "component"refers to the object-oriented notion of composition and not to architecture-level components. For example, you can modela person like this:

public class Person { private java.util.Date birthday; private Name name; private String key; public String getKey() { return key; } private void setKey(String key) { this.key=key; } public java.util.Date getBirthday() { return birthday; } public void setBirthday(java.util.Date birthday) { this.birthday = birthday; } public Name getName() { return name; } public void setName(Name name) { this.name = name; } ...... ......}

public class Name { char initial; String first; String last; public String getFirst() { return first; } void setFirst(String first) { this.first = first; } public String getLast() { return last; } void setLast(String last) { this.last = last; } public char getInitial() { return initial; } void setInitial(char initial) { this.initial = initial; }}

Now Name can be persisted as a component of Person. Name defines getter and setter methods for its persistent properties,

but it does not need to declare any interfaces or identifier properties.

Our Hibernate mapping would look like this:

<class name="eg.Person" table="person"> <id name="Key" column="pid" type="string"> <generator class="uuid"/> </id> <property name="birthday" type="date"/> <component name="Name" class="eg.Name">  <property name="initial"/> <property name="first"/> <property name="last"/> </component></class>

The person table would have the columns pid, birthday, initial, first and last.

Like value types, components do not support shared references. In other words, two persons could have the same name,but the two person objects would contain two independent name objects that were only "the same" by value. The nullvalue semantics of a component are ad hoc. When reloading the containing object, Hibernate will assume that if allcomponent columns are null, then the entire component is null. This is suitable for most purposes.

The properties of a component can be of any Hibernate type (collections, many-to-one associations, other components,etc). Nested components should not be considered an exotic usage. Hibernate is intended to support a fine-grained objectmodel.

The <component> element allows a <parent> subelement that maps a property of the component class as a reference back

to the containing entity.

<class name="eg.Person" table="person"> <id name="Key" column="pid" type="string"> <generator class="uuid"/> </id> <property name="birthday" type="date"/> <component name="Name" class="eg.Name" unique="true"> <parent name="namedPerson"/>  <property name="initial"/> <property name="first"/> <property name="last"/> </component></class>

9.2. Collections of dependent objects

Collections of components are supported (e.g. an array of type Name). Declare your component collection by replacing the

<element> tag with a <composite-element> tag:

<set name="someNames" table="some_names" lazy="true"> <key column="id"/> <composite-element class="eg.Name">  <property name="initial"/> <property name="first"/> <property name="last"/> </composite-element></set>

Important

If you define a Set of composite elements, it is important to implement equals() and

hashCode() correctly.

Composite elements can contain components but not collections. If your composite element contains components, use the<nested-composite-element> tag. This case is a collection of components which themselves have components. You may

want to consider if a one-to-many association is more appropriate. Remodel the composite element as an entity, but beaware that even though the Java model is the same, the relational model and persistence semantics are still slightlydifferent.

A composite element mapping does not support null-able properties if you are using a <set>. There is no separate primary

key column in the composite element table. Hibernate uses each column's value to identify a record when deletingobjects, which is not possible with null values. You have to either use only not-null properties in a composite-element orchoose a <list>, <map>, <bag> or <idbag>.

A special case of a composite element is a composite element with a nested <many-to-one> element. This mapping allows

you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association from Order to Item, where purchaseDate, price and quantity are properties of the association:

<class name="eg.Order" .... > .... <set name="purchasedItems" table="purchase_items" lazy="true"> <key column="order_id"> <composite-element class="eg.Purchase"> <property name="purchaseDate"/> <property name="price"/> <property name="quantity"/> <many-to-one name="item" class="eg.Item"/>  </composite-element> </set></class>

There cannot be a reference to the purchase on the other side for bidirectional association navigation. Components arevalue types and do not allow shared references. A single Purchase can be in the set of an Order, but it cannot be

referenced by the Item at the same time.

Even ternary (or quaternary, etc) associations are possible:

<class name="eg.Order" .... > .... <set name="purchasedItems" table="purchase_items" lazy="true"> <key column="order_id"> <composite-element class="eg.OrderLine"> <many-to-one name="purchaseDetails class="eg.Purchase"/> <many-to-one name="item" class="eg.Item"/> </composite-element> </set></class>

Composite elements can appear in queries using the same syntax as associations to other entities.

9.3. Components as Map indices

The <composite-map-key> element allows you to map a component class as the key of a Map. Ensure that you override

hashCode() and equals() correctly on the component class.

9.4. Components as composite identifiers

You can use a component as an identifier of an entity class. Your component class must satisfy certain requirements:

It must implement java.io.Serializable.

It must re-implement equals() and hashCode() consistently with the database's notion of composite key equality.

Note

In Hibernate, although the second requirement is not an absolutely hard requirement ofHibernate, it is recommended.

You cannot use an IdentifierGenerator to generate composite keys. Instead the application must assign its own identifiers.

Use the <composite-id> tag, with nested <key-property> elements, in place of the usual <id> declaration. For example, the

OrderLine class has a primary key that depends upon the (composite) primary key of Order.

<class name="OrderLine">

<composite-id name="id" class="OrderLineId"> <key-property name="lineId"/> <key-property name="orderId"/> <key-property name="customerId"/> </composite-id>

<property name="name"/>

<many-to-one name="order" class="Order" insert="false" update="false"> <column name="orderId"/> <column name="customerId"/> </many-to-one> ....

</class>

Any foreign keys referencing the OrderLine table are now composite. Declare this in your mappings for other classes. An

association to OrderLine is mapped like this:

<many-to-one name="orderLine" class="OrderLine"> <column name="lineId"/> <column name="orderId"/> <column name="customerId"/></many-to-one>

Tip

The column element is an alternative to the column attribute everywhere. Using the column

element just gives more declaration options, which are mostly useful when utilizinghbm2ddl

A many-to-many association to OrderLine also uses the composite foreign key:

<set name="undeliveredOrderLines"> <key column name="warehouseId"/> <many-to-many class="OrderLine"> <column name="lineId"/> <column name="orderId"/> <column name="customerId"/> </many-to-many></set>

The collection of OrderLines in Order would use:

<set name="orderLines" inverse="true"> <key> <column name="orderId"/> <column name="customerId"/> </key> <one-to-many class="OrderLine"/></set>

The <one-to-many> element declares no columns.

If OrderLine itself owns a collection, it also has a composite foreign key.

<class name="OrderLine"> .... .... <list name="deliveryAttempts"> <key>  <column name="lineId"/> <column name="orderId"/> <column name="customerId"/> </key> <list-index column="attemptId" base="1"/> <composite-element class="DeliveryAttempt"> ... </composite-element> </set></class>

9.5. Dynamic components

You can also map a property of type Map:

<dynamic-component name="userAttributes"> <property name="foo" column="FOO" type="string"/> <property name="bar" column="BAR" type="integer"/> <many-to-one name="baz" class="Baz" column="BAZ_ID"/></dynamic-component>

The semantics of a <dynamic-component> mapping are identical to <component>. The advantage of this kind of mapping is

the ability to determine the actual properties of the bean at deployment time just by editing the mapping document.Runtime manipulation of the mapping document is also possible, using a DOM parser. You can also access, and change,Hibernate's configuration-time metamodel via the Configuration object.

Chapter 10. Inheritance mapping

Table of Contents


10.1.1. Table per class hierarchy10.1.2. Table per subclass10.1.3. Table per subclass: using a discriminator10.1.4. Mixing table per class hierarchy with table per subclass10.1.5. Table per concrete class10.1.6. Table per concrete class using implicit polymorphism10.1.7. Mixing implicit polymorphism with other inheritance mappings

10.2. Limitations


Hibernate supports the three basic inheritance mapping strategies:

table per class hierarchy

table per subclass

table per concrete class

In addition, Hibernate supports a fourth, slightly different kind of polymorphism:

implicit polymorphism

It is possible to use different mapping strategies for different branches of the same inheritance hierarchy. You can thenmake use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However, Hibernate does notsupport mixing <subclass>, <joined-subclass> and <union-subclass> mappings under the same root <class> element. It is

possible to mix together the table per hierarchy and table per subclass strategies under the the same <class> element, by

combining the <subclass> and <join> elements (see below for an example).

It is possible to define subclass, union-subclass, and joined-subclass mappings in separate mapping documents directly

beneath hibernate-mapping. This allows you to extend a class hierarchy by adding a new mapping file. You must specify an

extends attribute in the subclass mapping, naming a previously mapped superclass. Previously this feature made the

ordering of the mapping documents important. Since Hibernate, the ordering of mapping files is irrelevant when using the

extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses.

<hibernate-mapping> <subclass name="DomesticCat" extends="Cat" discriminator-value="D"> <property name="name" type="string"/> </subclass> </hibernate-mapping>

10.1.1. Table per class hierarchy

Suppose we have an interface Payment with the implementors CreditCardPayment, CashPayment, and ChequePayment. The

table per hierarchy mapping would display in the following way:

<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <property name="creditCardType" column="CCTYPE"/> ... </subclass> <subclass name="CashPayment" discriminator-value="CASH"> ... </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> ... </subclass></class>

Exactly one table is required. There is a limitation of this mapping strategy: columns declared by the subclasses, such asCCTYPE, cannot have NOT NULL constraints.

10.1.2. Table per subclass

A table per subclass mapping looks like this:

<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="AMOUNT"/> ... <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> <key column="PAYMENT_ID"/> <property name="creditCardType" column="CCTYPE"/> ... </joined-subclass> <joined-subclass name="CashPayment" table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> ... </joined-subclass> <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> <key column="PAYMENT_ID"/> ... </joined-subclass></class>

Four tables are required. The three subclass tables have primary key associations to the superclass table so the relationalmodel is actually a one-to-one association.

10.1.3. Table per subclass: using a discriminator

Hibernate's implementation of table per subclass does not require a discriminator column. Other object/relational mappersuse a different implementation of table per subclass that requires a type discriminator column in the superclass table. Theapproach taken by Hibernate is much more difficult to implement, but arguably more correct from a relational point of view.If you want to use a discriminator column with the table per subclass strategy, you can combine the use of <subclass> and

<join>, as follows:

<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <join table="CREDIT_PAYMENT"> <key column="PAYMENT_ID"/> <property name="creditCardType" column="CCTYPE"/> ... </join> </subclass> <subclass name="CashPayment" discriminator-value="CASH"> <join table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> ... </join> </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> <join table="CHEQUE_PAYMENT" fetch="select"> <key column="PAYMENT_ID"/> ... </join> </subclass></class>

The optional fetch="select" declaration tells Hibernate not to fetch the ChequePayment subclass data using an outer join

when querying the superclass.

10.1.4. Mixing table per class hierarchy with table per subclass

You can even mix the table per hierarchy and table per subclass strategies using the following approach:

<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <join table="CREDIT_PAYMENT"> <property name="creditCardType" column="CCTYPE"/> ... </join> </subclass> <subclass name="CashPayment" discriminator-value="CASH"> ... </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> ... </subclass></class>

For any of these mapping strategies, a polymorphic association to the root Payment class is mapped using <many-to-one>.

<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>

10.1.5. Table per concrete class

There are two ways we can map the table per concrete class strategy. First, you can use <union-subclass>.

<class name="Payment"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="sequence"/> </id> <property name="amount" column="AMOUNT"/> ... <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> <property name="creditCardType" column="CCTYPE"/> ... </union-subclass> <union-subclass name="CashPayment" table="CASH_PAYMENT"> ... </union-subclass> <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> ... </union-subclass></class>

Three tables are involved for the subclasses. Each table defines columns for all properties of the class, including inheritedproperties.

The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on allsubclass tables. The identity generator strategy is not allowed in union subclass inheritance. The primary key seed has tobe shared across all unioned subclasses of a hierarchy.

If your superclass is abstract, map it with abstract="true". If it is not abstract, an additional table (it defaults to PAYMENT in the

example above), is needed to hold instances of the superclass.

10.1.6. Table per concrete class using implicit polymorphism

An alternative approach is to make use of implicit polymorphism:

<class name="CreditCardPayment" table="CREDIT_PAYMENT"> <id name="id" type="long" column="CREDIT_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CREDIT_AMOUNT"/> ...</class>

<class name="CashPayment" table="CASH_PAYMENT"> <id name="id" type="long" column="CASH_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CASH_AMOUNT"/> ...</class>

<class name="ChequePayment" table="CHEQUE_PAYMENT"> <id name="id" type="long" column="CHEQUE_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CHEQUE_AMOUNT"/> ...</class>

Notice that the Payment interface is not mentioned explicitly. Also notice that properties of Payment are mapped in each of

the subclasses. If you want to avoid duplication, consider using XML entities (for example,[ <!ENTITY allproperties SYSTEM "allproperties.xml"> ] in the DOCTYPE declaration and %allproperties; in the mapping).

The disadvantage of this approach is that Hibernate does not generate SQL UNIONs when performing polymorphic

queries.

For this mapping strategy, a polymorphic association to Payment is usually mapped using <any>.

<any name="payment" meta-type="string" id-type="long"> <meta-value value="CREDIT" class="CreditCardPayment"/> <meta-value value="CASH" class="CashPayment"/> <meta-value value="CHEQUE" class="ChequePayment"/> <column name="PAYMENT_CLASS"/> <column name="PAYMENT_ID"/></any>

10.1.7. Mixing implicit polymorphism with other inheritance mappings

Since the subclasses are each mapped in their own <class> element, and since Payment is just an interface), each of the

subclasses could easily be part of another inheritance hierarchy. You can still use polymorphic queries against thePayment interface.

<class name="CreditCardPayment" table="CREDIT_PAYMENT"> <id name="id" type="long" column="CREDIT_PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="CREDIT_CARD" type="string"/> <property name="amount" column="CREDIT_AMOUNT"/> ... <subclass name="MasterCardPayment" discriminator-value="MDC"/> <subclass name="VisaPayment" discriminator-value="VISA"/></class>

<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN"> <id name="id" type="long" column="TXN_ID"> <generator class="native"/> </id> ... <joined-subclass name="CashPayment" table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> <property name="amount" column="CASH_AMOUNT"/> ... </joined-subclass> <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> <key column="PAYMENT_ID"/> <property name="amount" column="CHEQUE_AMOUNT"/> ... </joined-subclass></class>

Once again, Payment is not mentioned explicitly. If we execute a query against the Payment interface, for example

from Payment, Hibernate automatically returns instances of CreditCardPayment (and its subclasses, since they also

implement Payment), CashPayment and ChequePayment, but not instances of NonelectronicTransaction.

10.2. Limitations

There are limitations to the "implicit polymorphism" approach to the table per concrete-class mapping strategy. There aresomewhat less restrictive limitations to <union-subclass> mappings.

The following table shows the limitations of table per concrete-class mappings, and of implicit polymorphism, in Hibernate.

Table 10.1. Features of inheritance mappings

Inheritancestrategy

Polymorphicmany-to-one

Polymorphicone-to-one

Polymorphicone-to-many

Polymorphicmany-to-many

Polymorphic load()/get()Polymorphicqueries

table perclass-hierarchy

<many-to-one> <one-to-one> <one-to-many> <many-to-many> s.get(Payment.class, id) from Payment p

table persubclass

<many-to-one> <one-to-one> <one-to-many> <many-to-many> s.get(Payment.class, id) from Payment p

table perconcrete-class(union-subclass)

<many-to-one> <one-to-one>

<one-to-many>

(forinverse="true"

only)

<many-to-many> s.get(Payment.class, id) from Payment p

table perconcrete class(implicitpolymorphism)

<any>notsupported

notsupported

<many-to-any> s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult() from Payment p

Chapter 11. Working with objects

Table of Contents

11.1. Hibernate object states11.2. Making objects persistent11.3. Loading an object11.4. Querying

11.4.1. Executing queries11.4.2. Filtering collections11.4.3. Criteria queries11.4.4. Queries in native SQL

11.5. Modifying persistent objects11.6. Modifying detached objects11.7. Automatic state detection11.8. Deleting persistent objects11.9. Replicating object between two different datastores

11.10. Flushing the Session11.11. Transitive persistence11.12. Using metadata

Hibernate is a full object/relational mapping solution that not only shields the developer from the details of the underlyingdatabase management system, but also offers state management of objects. This is, contrary to the management of SQLstatements in common JDBC/SQL persistence layers, a natural object-oriented view of persistence in Java applications.

In other words, Hibernate application developers should always think about the state of their objects, and not necessarilyabout the execution of SQL statements. This part is taken care of by Hibernate and is only relevant for the applicationdeveloper when tuning the performance of the system.

11.1. Hibernate object states

Hibernate defines and supports the following object states:

Transient - an object is transient if it has just been instantiated using the new operator, and it is not associated with a

Hibernate Session. It has no persistent representation in the database and no identifier value has been assigned.

Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore.Use the Hibernate Session to make an object persistent (and let Hibernate take care of the SQL statements that need to

be executed for this transition).

Persistent - a persistent instance has a representation in the database and an identifier value. It might just have beensaved or loaded, however, it is by definition in the scope of a Session. Hibernate will detect any changes made to an

object in persistent state and synchronize the state with the database when the unit of work completes. Developers donot execute manual UPDATE statements, or DELETE statements when an object should be made transient.

Detached - a detached instance is an object that has been persistent, but its Session has been closed. The reference to

the object is still valid, of course, and the detached instance might even be modified in this state. A detached instancecan be reattached to a new Session at a later point in time, making it (and all the modifications) persistent again. This

feature enables a programming model for long running units of work that require user think-time. We call themapplication transactions, i.e., a unit of work from the point of view of the user.

We will now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.

11.2. Making objects persistent

Newly instantiated instances of a persistent class are considered transient by Hibernate. We can make a transientinstance persistent by associating it with a session:

DomesticCat fritz = new DomesticCat();fritz.setColor(Color.GINGER);fritz.setSex('M');fritz.setName("Fritz");Long generatedId = (Long) sess.save(fritz);

If Cat has a generated identifier, the identifier is generated and assigned to the cat when save() is called. If Cat has an

assigned identifier, or a composite key, the identifier should be assigned to the cat instance before calling save(). You can

also use persist() instead of save(), with the semantics defined in the EJB3 early draft.

persist() makes a transient instance persistent. However, it does not guarantee that the identifier value will be assigned

to the persistent instance immediately, the assignment might happen at flush time. persist() also guarantees that it will

not execute an INSERT statement if it is called outside of transaction boundaries. This is useful in long-running

conversations with an extended Session/persistence context.

save() does guarantee to return an identifier. If an INSERT has to be executed to get the identifier ( e.g. "identity"

generator, not "sequence"), this INSERT happens immediately, no matter if you are inside or outside of a transaction.This is problematic in a long-running conversation with an extended Session/persistence context.

Alternatively, you can assign the identifier using an overloaded version of save().

DomesticCat pk = new DomesticCat();pk.setColor(Color.TABBY);pk.setSex('F');pk.setName("PK");pk.setKittens( new HashSet() );pk.addKitten(fritz);sess.save( pk, new Long(1234) );

If the object you make persistent has associated objects (e.g. the kittens collection in the previous example), these objects

can be made persistent in any order you like unless you have a NOT NULL constraint upon a foreign key column. There is

never a risk of violating foreign key constraints. However, you might violate a NOT NULL constraint if you save() the objects

in the wrong order.

Usually you do not bother with this detail, as you will normally use Hibernate's transitive persistence feature to save theassociated objects automatically. Then, even NOT NULL constraint violations do not occur - Hibernate will take care of

everything. Transitive persistence is discussed later in this chapter.

11.3. Loading an object

The load() methods of Session provide a way of retrieving a persistent instance if you know its identifier. load() takes a class

object and loads the state into a newly instantiated instance of that class in a persistent state.

Cat fritz = (Cat) sess.load(Cat.class, generatedId);

// you need to wrap primitive identifierslong id = 1234;DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );

Alternatively, you can load state into a given instance:

Cat cat = new DomesticCat();// load pk's state into catsess.load( cat, new Long(pkId) );Set kittens = cat.getKittens();

Be aware that load() will throw an unrecoverable exception if there is no matching database row. If the class is mapped

with a proxy, load() just returns an uninitialized proxy and does not actually hit the database until you invoke a method of

the proxy. This is useful if you wish to create an association to an object without actually loading it from the database. Italso allows multiple instances to be loaded as a batch if batch-size is defined for the class mapping.

If you are not certain that a matching row exists, you should use the get() method which hits the database immediately and

returns null if there is no matching row.

Cat cat = (Cat) sess.get(Cat.class, id);if (cat==null) { cat = new Cat(); sess.save(cat, id);}return cat;

You can even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. See the API documentation for

more information.

Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);

Any associated instances or contained collections will not be selected FOR UPDATE, unless you decide to specify lock or all

as a cascade style for the association.

It is possible to re-load an object and all its collections at any time, using the refresh() method. This is useful when

database triggers are used to initialize some of the properties of the object.

sess.save(cat);sess.flush(); //force the SQL INSERTsess.refresh(cat); //re-read the state (after the trigger executes)

How much does Hibernate load from the database and how many SQL SELECTs will it use? This depends on the fetching

strategy. This is explained in Section 20.1, “Fetching strategies”.

11.4. Querying

If you do not know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-usebut powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticatedCriteria and Example query feature (QBC and QBE). You can also express your query in the native SQL of your database,with optional support from Hibernate for result set conversion into objects.

11.4.1. Executing queries

HQL and native SQL queries are represented with an instance of org.hibernate.Query. This interface offers methods for

parameter binding, result set handling, and for the execution of the actual query. You always obtain a Query using the

current Session:

List cats = session.createQuery( "from Cat as cat where cat.birthdate < ?") .setDate(0, date) .list();

List mothers = session.createQuery( "select mother from Cat as cat join cat.mother as mother where cat.name = ?") .setString(0, name) .list();

List kittens = session.createQuery( "from Cat as cat where cat.mother = ?") .setEntity(0, pk) .list();

Cat mother = (Cat) session.createQuery( "select cat.mother from Cat as cat where cat = ?") .setEntity(0, izi) .uniqueResult();]]

Query mothersWithKittens = (Cat) session.createQuery( "select mother from Cat as mother left join fetch mother.kittens");Set uniqueMothers = new HashSet(mothersWithKittens.list());

A query is usually executed by invoking list(). The result of the query will be loaded completely into a collection in memory.

Entity instances retrieved by a query are in a persistent state. The uniqueResult() method offers a shortcut if you know your

query will only return a single object. Queries that make use of eager fetching of collections usually return duplicates of theroot objects, but with their collections initialized. You can filter these duplicates through a Set.

11.4.1.1. Iterating results

Occasionally, you might be able to achieve better performance by executing the query using the iterate() method. This will

usually be the case if you expect that the actual entity instances returned by the query will already be in the session orsecond-level cache. If they are not already cached, iterate() will be slower than list() and might require many database hits

for a simple query, usually 1 for the initial select which only returns identifiers, and n additional selects to initialize theactual instances.

// fetch idsIterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate();while ( iter.hasNext() ) { Qux qux = (Qux) iter.next(); // fetch the object // something we couldnt express in the query if ( qux.calculateComplicatedAlgorithm() ) { // delete the current instance iter.remove(); // dont need to process the rest break; }}

11.4.1.2. Queries that return tuples

Hibernate queries sometimes return tuples of objects. Each tuple is returned as an array:

Iterator kittensAndMothers = sess.createQuery( "select kitten, mother from Cat kitten join kitten.mother mother") .list() .iterator();

while ( kittensAndMothers.hasNext() ) { Object[] tuple = (Object[]) kittensAndMothers.next(); Cat kitten = (Cat) tuple[0]; Cat mother = (Cat) tuple[1]; ....}

11.4.1.3. Scalar results

Queries can specify a property of a class in the select clause. They can even call SQL aggregate functions. Properties or

aggregates are considered "scalar" results and not entities in persistent state.

Iterator results = sess.createQuery( "select cat.color, min(cat.birthdate), count(cat) from Cat cat " + "group by cat.color") .list() .iterator();

while ( results.hasNext() ) { Object[] row = (Object[]) results.next(); Color type = (Color) row[0]; Date oldest = (Date) row[1]; Integer count = (Integer) row[2]; .....}

11.4.1.4. Bind parameters

Methods on Query are provided for binding values to named parameters or JDBC-style ? parameters. Contrary to JDBC,

Hibernate numbers parameters from zero. Named parameters are identifiers of the form :name in the query string. The

advantages of named parameters are as follows:

named parameters are insensitive to the order they occur in the query string

they can occur multiple times in the same query

they are self-documenting

//named parameter (preferred)Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");q.setString("name", "Fritz");Iterator cats = q.iterate();

//positional parameterQuery q = sess.createQuery("from DomesticCat cat where cat.name = ?");q.setString(0, "Izi");Iterator cats = q.iterate();

//named parameter listList names = new ArrayList();names.add("Izi");names.add("Fritz");Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)");q.setParameterList("namesList", names);List cats = q.list();

11.4.1.5. Pagination

If you need to specify bounds upon your result set, that is, the maximum number of rows you want to retrieve and/or thefirst row you want to retrieve, you can use methods of the Query interface:

Query q = sess.createQuery("from DomesticCat cat");q.setFirstResult(20);q.setMaxResults(10);List cats = q.list();

Hibernate knows how to translate this limit query into the native SQL of your DBMS.

11.4.1.6. Scrollable iteration

If your JDBC driver supports scrollable ResultSets, the Query interface can be used to obtain a ScrollableResults object that

allows flexible navigation of the query results.

Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " + "order by cat.name");ScrollableResults cats = q.scroll();if ( cats.first() ) {

// find the first name on each page of an alphabetical list of cats by name firstNamesOfPages = new ArrayList(); do { String name = cats.getString(0); firstNamesOfPages.add(name); } while ( cats.scroll(PAGE_SIZE) );

// Now get the first page of cats pageOfCats = new ArrayList(); cats.beforeFirst(); int i=0; while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) );

}cats.close()

Note that an open database connection and cursor is required for this functionality. Use setMaxResult()/setFirstResult() if you

need offline pagination functionality.

11.4.1.7. Externalizing named queries

Queries can also be configured as so called named queries using annotations or Hibernate mapping documents.@NamedQuery and @NamedQueries can be defined at the class level as seen in Example 11.1, “Defining a named query

using @NamedQuery” . However their definitions are global to the session factory/entity manager factory scope. A named

query is defined by its name and the actual query string.

Example 11.1. Defining a named query using @NamedQuery

@Entity@NamedQuery(name="night.moreRecentThan", query="select n from Night n where n.date >= :date")public class Night { ...}

public class MyDao { doStuff() { Query q = s.getNamedQuery("night.moreRecentThan"); q.setDate( "date", aMonthAgo ); List results = q.list(); ... } ...}

Using a mapping document can be configured using the <query> node. Remember to use a CDATA section if your query

contains characters that could be interpreted as markup.

Example 11.2. Defining a named query using <query>

<query name="ByNameAndMaximumWeight"><![CDATA[ from eg.DomesticCat as cat where cat.name = ? and cat.weight > ?] ]></query>

Parameter binding and executing is done programatically as seen i n Example 11.3, “Parameter binding of a namedquery”.

Example 11.3. Parameter binding of a named query

Query q = sess.getNamedQuery("ByNameAndMaximumWeight");q.setString(0, name);q.setInt(1, minWeight);List cats = q.list();

The actual program code is independent of the query language that is used. You can also define native SQL queries inmetadata, or migrate existing queries to Hibernate by placing them in mapping files.

Also note that a query declaration inside a <hibernate-mapping> element requires a global unique name for the query, while

a query declaration inside a <class> element is made unique automatically by prepending the fully qualified name of the

class. For example eg.Cat.ByNameAndMaximumWeight.

11.4.2. Filtering collections

A collection filter is a special type of query that can be applied to a persistent collection or array. The query string can referto this, meaning the current collection element.

Collection blackKittens = session.createFilter( pk.getKittens(), "where this.color = ?") .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) ) .list());

The returned collection is considered a bag that is a copy of the given collection. The original collection is not modified.This is contrary to the implication of the name "filter", but consistent with expected behavior.

Observe that filters do not require a from clause, although they can have one if required. Filters are not limited to returning

the collection elements themselves.

Collection blackKittenMates = session.createFilter( pk.getKittens(), "select this.mate where this.color = eg.Color.BLACK.intValue") .list();

Even an empty filter query is useful, e.g. to load a subset of elements in a large collection:

Collection tenKittens = session.createFilter( mother.getKittens(), "") .setFirstResult(0).setMaxResults(10) .list();

11.4.3. Criteria queries

HQL is extremely powerful, but some developers prefer to build queries dynamically using an object-oriented API, ratherthan building query strings. Hibernate provides an intuitive Criteria query API for these cases:

Criteria crit = session.createCriteria(Cat.class);crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );crit.setMaxResults(10);List cats = crit.list();

The Criteria and the associated Example API are discussed in more detail in Chapter 17, Criteria Queries.

11.4.4. Queries in native SQL

You can express a query in SQL, using createSQLQuery() and let Hibernate manage the mapping from result sets to

objects. You can at any time call session.connection() and use the JDBC Connection directly. If you choose to use the

Hibernate API, you must enclose SQL aliases in braces:

List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class).list();

List cats = session.createSQLQuery( "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " + "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " + "FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class).list()

SQL queries can contain named and positional parameters, just like Hibernate queries. More information about nativeSQL queries in Hibernate can be found in Chapter 18, Native SQL.

11.5. Modifying persistent objects

Transactional persistent instances (i.e. objects loaded, saved, created or queried by the Session) can be manipulated by

the application, and any changes to persistent state will be persisted when the Session is flushed. This is discussed later in

this chapter. There is no need to call a particular method (like update(), which has a different purpose) to make your

modifications persistent. The most straightforward way to update the state of an object is to load() it and then manipulate it

directly while the Session is open:

DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );cat.setName("PK");sess.flush(); // changes to cat are automatically detected and persisted

Sometimes this programming model is inefficient, as it requires in the same session both an SQL SELECT to load an object

and an SQL UPDATE to persist its updated state. Hibernate offers an alternate approach by using detached instances.

11.6. Modifying detached objects

Many applications need to retrieve an object in one transaction, send it to the UI layer for manipulation, then save thechanges in a new transaction. Applications that use this kind of approach in a high-concurrency environment usually useversioned data to ensure isolation for the "long" unit of work.

Hibernate supports this model by providing for reattachment of detached instances using the Session.update() or

Session.merge() methods:

// in the first sessionCat cat = (Cat) firstSession.load(Cat.class, catId);Cat potentialMate = new Cat();firstSession.save(potentialMate);

// in a higher layer of the applicationcat.setMate(potentialMate);

// later, in a new sessionsecondSession.update(cat); // update catsecondSession.update(mate); // update mate

If the Cat with identifier catId had already been loaded by secondSession when the application tried to reattach it, an

exception would have been thrown.

Use update() if you are certain that the session does not contain an already persistent instance with the same identifier.

Use merge() if you want to merge your modifications at any time without consideration of the state of the session. In other

words, update() is usually the first method you would call in a fresh session, ensuring that the reattachment of your

detached instances is the first operation that is executed.

The application should individually update() detached instances that are reachable from the given detached instance only

if it wants their state to be updated. This can be automated using transitive persistence. See Section 11.11, “Transitivepersistence” for more information.

The lock() method also allows an application to reassociate an object with a new session. However, the detached instance

has to be unmodified.

//just reassociate:sess.lock(fritz, LockMode.NONE);//do a version check, then reassociate:sess.lock(izi, LockMode.READ);//do a version check, using SELECT ... FOR UPDATE, then reassociate:sess.lock(pk, LockMode.UPGRADE);

Note that lock() can be used with various LockModes. See the API documentation and the chapter on transaction handling

for more information. Reattachment is not the only usecase for lock().

Other models for long units of work are discussed in Section 13.3, “Optimistic concurrency control”.

11.7. Automatic state detection

Hibernate users have requested a general purpose method that either saves a transient instance by generating a newidentifier or updates/reattaches the detached instances associated with its current identifier. The saveOrUpdate() method

implements this functionality.

// in the first sessionCat cat = (Cat) firstSession.load(Cat.class, catID);

// in a higher tier of the applicationCat mate = new Cat();cat.setMate(mate);

// later, in a new sessionsecondSession.saveOrUpdate(cat); // update existing state (cat has a non-null id)secondSession.saveOrUpdate(mate); // save the new instance (mate has a null id)

The usage and semantics of saveOrUpdate() seems to be confusing for new users. Firstly, so long as you are not trying to

use instances from one session in another new session, you should not need to use update(), saveOrUpdate(), or merge().

Some whole applications will never use either of these methods.

Usually update() or saveOrUpdate() are used in the following scenario:

the application loads an object in the first session

the object is passed up to the UI tier

some modifications are made to the object

the object is passed back down to the business logic tier

the application persists these modifications by calling update() in a second session

saveOrUpdate() does the following:

if the object is already persistent in this session, do nothing

if another object associated with the session has the same identifier, throw an exception

if the object has no identifier property, save() it

if the object's identifier has the value assigned to a newly instantiated object, save() it

if the object is versioned by a <version> or <timestamp>, and the version property value is the same value assigned to a

newly instantiated object, save() it

otherwise update() the object

and merge() is very different:

if there is a persistent instance with the same identifier currently associated with the session, copy the state of thegiven object onto the persistent instance

if there is no persistent instance currently associated with the session, try to load it from the database, or create a newpersistent instance

the persistent instance is returned

the given instance does not become associated with the session, it remains detached

11.8. Deleting persistent objects

Session.delete() will remove an object's state from the database. Your application, however, can still hold a reference to a

deleted object. It is best to think of delete() as making a persistent instance, transient.

sess.delete(cat);

You can delete objects in any order, without risk of foreign key constraint violations. It is still possible to violate a NOT NULL

constraint on a foreign key column by deleting objects in the wrong order, e.g. if you delete the parent, but forget to deletethe children.

11.9. Replicating object between two different datastores

It is sometimes useful to be able to take a graph of persistent instances and make them persistent in a different datastore,without regenerating identifier values.

//retrieve a cat from one databaseSession session1 = factory1.openSession();Transaction tx1 = session1.beginTransaction();Cat cat = session1.get(Cat.class, catId);tx1.commit();session1.close();

//reconcile with a second databaseSession session2 = factory2.openSession();Transaction tx2 = session2.beginTransaction();session2.replicate(cat, ReplicationMode.LATEST_VERSION);tx2.commit();session2.close();

The ReplicationMode determines how replicate() will deal with conflicts with existing rows in the database:

ReplicationMode.IGNORE: ignores the object when there is an existing database row with the same identifier

ReplicationMode.OVERWRITE: overwrites any existing database row with the same identifier

ReplicationMode.EXCEPTION: throws an exception if there is an existing database row with the same identifier

ReplicationMode.LATEST_VERSION: overwrites the row if its version number is earlier than the version number of the

object, or ignore the object otherwise

Usecases for this feature include reconciling data entered into different database instances, upgrading systemconfiguration information during product upgrades, rolling back changes made during non-ACID transactions and more.

11.10. Flushing the Session

Sometimes the Session will execute the SQL statements needed to synchronize the JDBC connection's state with the state

of objects held in memory. This process, called flush, occurs by default at the following points:

before some query executions

from org.hibernate.Transaction.commit()

from Session.flush()

The SQL statements are issued in the following order:

1. all entity insertions in the same order the corresponding objects were saved using Session.save()

2. all entity updates

3. all collection deletions

4. all collection element deletions, updates and insertions

5. all collection insertions

6. all entity deletions in the same order the corresponding objects were deleted using Session.delete()

An exception is that objects using native ID generation are inserted when they are saved.

Except when you explicitly flush(), there are absolutely no guarantees about when the Session executes the JDBC calls,

only the order in which they are executed. However, Hibernate does guarantee that the Query.list(..) will never return stale

or incorrect data.

It is possible to change the default behavior so that flush occurs less frequently. The FlushMode class defines three different

modes: only flush at commit time when the Hibernate Transaction API is used, flush automatically using the explained

routine, or never flush unless flush() is called explicitly. The last mode is useful for long running units of work, where a

Session is kept open and disconnected for a long time (see Section 13.3.2, “Extended session and automatic versioning”).

sess = sf.openSession();Transaction tx = sess.beginTransaction();sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state

Cat izi = (Cat) sess.load(Cat.class, id);izi.setName(iznizi);

// might return stale datasess.find("from Cat as cat left outer join cat.kittens kitten");

// change to izi is not flushed!...tx.commit(); // flush occurssess.close();

During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling exceptions involvessome understanding of Hibernate's transactional behavior, we discuss it in Chapter 13, Transactions and Concurrency.

11.11. Transitive persistence

It is quite cumbersome to save, delete, or reattach individual objects, especially if you deal with a graph of associatedobjects. A common case is a parent/child relationship. Consider the following example:

If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cyclewould depend on the parent and no further action would be required for convenient "cascading" of state changes. Whenthe parent is saved, the value-typed child objects are saved and when the parent is deleted, the children will be deleted,etc. This works for operations such as the removal of a child from the collection. Since value-typed objects cannot haveshared references, Hibernate will detect this and delete the child from the database.

Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items,or parent and child cats). Entities have their own life cycle and support shared references. Removing an entity from thecollection does not mean it can be deleted), and there is by default no cascading of state from one entity to any otherassociated entities. Hibernate does not implement persistence by reachability by default.

For each basic operation of the Hibernate session - including persist(), merge(), saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate() - there is a corresponding cascade style.

Respectively, the cascade styles are named create, merge, save-update, delete, lock, refresh, evict, replicate. If you want an

operation to be cascaded along an association, you must indicate that in the mapping document. For example:

<one-to-one name="person" cascade="persist"/>

Cascade styles my be combined:

<one-to-one name="person" cascade="persist,delete,lock"/>

You can even use cascade="all" to specify that all operations should be cascaded along the association. The default

cascade="none" specifies that no operations are to be cascaded.

In case you are using annotatons you probably have noticed the cascade attribute taking an array of CascadeType as a

value. The cascade concept in JPA is very is similar to the transitive persistence and cascading of operations as describedabove, but with slightly different semantics and cascading types:

CascadeType.PERSIST: cascades the persist (create) operation to associated entities persist() is called or if the entity is

managed

CascadeType.MERGE: cascades the merge operation to associated entities if merge() is called or if the entity is

managed

CascadeType.REMOVE: cascades the remove operation to associated entities if delete() is called

CascadeType.REFRESH: cascades the refresh operation to associated entities if refresh() is called

CascadeType.DETACH: cascades the detach operation to associated entities if detach() is called

CascadeType.ALL: all of the above

Note

CascadeType.ALL also covers Hibernate specific operations like save-update, lock etc...

A special cascade style, delete-orphan, applies only to one-to-many associations, and indicates that the delete() operation

should be applied to any child object that is removed from the association. Using annotations there is noCascadeType.DELETE-ORPHAN equivalent. Instead you can use the attribute orphanRemoval as seen in Example 11.4,

“@OneToMany with orphanRemoval”. If an entity is removed from a @OneToMany collection or an associated entity is

dereferenced from a @OneToOne association, this associated entity can be marked for deletion if orphanRemoval is set to

true.

Example 11.4. @OneToMany with orphanRemoval

@Entity public class Customer { private Set<Order> orders;

@OneToMany(cascade=CascadeType.ALL, orphanRemoval=true) public Set<Order> getOrders() { return orders; }

public void setOrders(Set<Order> orders) { this.orders = orders; }

[...]}

@Entity public class Order { ... }

Customer customer = em.find(Customer.class, 1l);Order order = em.find(Order.class, 1l);customer.getOrders().remove(order); //order will be deleted by cascade

Recommendations:

It does not usually make sense to enable cascade on a many-to-one or many-to-many association. In fact the@ManyToOne and @ManyToMany don't even offer a orphanRemoval attribute. Cascading is often useful for one-to-one

and one-to-many associations.

If the child object's lifespan is bounded by the lifespan of the parent object, make it a life cycle object by specifyingcascade="all,delete-orphan" (@OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)).

Otherwise, you might not need cascade at all. But if you think that you will often be working with the parent andchildren together in the same transaction, and you want to save yourself some typing, consider usingcascade="persist,merge,save-update".

Mapping an association (either a single valued association, or a collection) with cascade="all" marks the association as a

parent/child style relationship where save/update/delete of the parent results in save/update/delete of the child or children.

Furthermore, a mere reference to a child from a persistent parent will result in save/update of the child. This metaphor isincomplete, however. A child which becomes unreferenced by its parent is not automatically deleted, except in the case of

a one-to-many association mapped with cascade="delete-orphan". The precise semantics of cascading operations for a

parent/child relationship are as follows:

If a parent is passed to persist(), all children are passed to persist()

If a parent is passed to merge(), all children are passed to merge()

If a parent is passed to save(), update() or saveOrUpdate(), all children are passed to saveOrUpdate()

If a transient or detached child becomes referenced by a persistent parent, it is passed to saveOrUpdate()

If a parent is deleted, all children are passed to delete()

If a child is dereferenced by a persistent parent, nothing special happens - the application should explicitly delete thechild if necessary - unless cascade="delete-orphan", in which case the "orphaned" child is deleted.

Finally, note that cascading of operations can be applied to an object graph at call time or at flush time. All operations, ifenabled, are cascaded to associated entities reachable when the operation is executed. However, save-update and

delete-orphan are transitive for all associated entities reachable during flush of the Session.

11.12. Using metadata

Hibernate requires a rich meta-level model of all entity and value types. This model can be useful to the application itself.For example, the application might use Hibernate's metadata to implement a "smart" deep-copy algorithm thatunderstands which objects should be copied (eg. mutable value types) and which objects that should not (e.g. immutablevalue types and, possibly, associated entities).

Hibernate exposes metadata via the ClassMetadata and CollectionMetadata interfaces and the Type hierarchy. Instances of

the metadata interfaces can be obtained from the SessionFactory.

Cat fritz = ......;ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);

Object[] propertyValues = catMeta.getPropertyValues(fritz);String[] propertyNames = catMeta.getPropertyNames();Type[] propertyTypes = catMeta.getPropertyTypes();

// get a Map of all properties which are not collections or associationsMap namedValues = new HashMap();for ( int i=0; i<propertyNames.length; i++ ) { if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) { namedValues.put( propertyNames[i], propertyValues[i] ); }}

Chapter 12. Read-only entities

Table of Contents


12.1.1. Entities of immutable classes12.1.2. Loading persistent entities as read-only12.1.3. Loading read-only entities from an HQL query/criteria12.1.4. Making a persistent entity read-only


12.2.1. Simple properties12.2.2. Unidirectional associations12.2.3. Bidirectional associations

Important

Hibernate's treatment of read-only entities may differ from what you may haveencountered elsewhere. Incorrect usage may cause unexpected results.

When an entity is read-only:

Hibernate does not dirty-check the entity's simple properties or single-ended associations;

Hibernate will not update simple properties or updatable single-ended associations;

Hibernate will not update the version of the read-only entity if only simple properties or single-ended updatableassociations are changed;

In some ways, Hibernate treats read-only entities the same as entities that are not read-only:

Hibernate cascades operations to associations as defined in the entity mapping.

Hibernate updates the version if the entity has a collection with changes that dirties the entity;

A read-only entity can be deleted.

Even if an entity is not read-only, its collection association can be affected if it contains a read-only entity.

For details about the affect of read-only entities on different property and association types, see Section 12.2, “Read-onlyaffect on property type”.

For details about how to make entities read-only, see Section 12.1, “Making persistent entities read-only”

Hibernate does some optimizing for read-only entities:

It saves execution time by not dirty-checking simple properties or single-ended associations.

It saves memory by deleting database snapshots.


Only persistent entities can be made read-only. Transient and detached entities must be put in persistent state before theycan be made read-only.

Hibernate provides the following ways to make persistent entities read-only:

you can map an entity class as immutable; when an entity of an immutable class is made persistent, Hibernateautomatically makes it read-only. see Section 12.1.1, “Entities of immutable classes” for details

you can change a default so that entities loaded into the session by Hibernate are automatically made read-only; seeSection 12.1.2, “Loading persistent entities as read-only” for details

you can make an HQL query or criteria read-only so that entities loaded when the query or criteria executes, scrolls, oriterates, are automatically made read-only; see Section 12.1.3, “Loading read-only entities from an HQL query/criteria”for details

you can make a persistent entity that is already in the in the session read-only; see Section 12.1.4, “Making apersistent entity read-only” for details

12.1.1. Entities of immutable classes

When an entity instance of an immutable class is made persistent, Hibernate automatically makes it read-only.

An entity of an immutable class can created and deleted the same as an entity of a mutable class.

Hibernate treats a persistent entity of an immutable class the same way as a read-only persistent entity of a mutable class.The only exception is that Hibernate will not allow an entity of an immutable class to be changed so it is not read-only.

12.1.2. Loading persistent entities as read-only

Note

Entities of immutable classes are automatically loaded as read-only.

To change the default behavior so Hibernate loads entity instances of mutable classes into the session and automaticallymakes them read-only, call:

Session.setDefaultReadOnly( true );

To change the default back so entities loaded by Hibernate are not made read-only, call:

Session.setDefaultReadOnly( false );

You can determine the current setting by calling:

Session.isDefaultReadOnly();

If Session.isDefaultReadOnly() returns true, entities loaded by the following are automatically made read-only:

Session.load()

Session.get()

Session.merge()

executing, scrolling, or iterating HQL queries and criteria; to override this setting for a particular HQL query or criteriasee Section 12.1.3, “Loading read-only entities from an HQL query/criteria”

Changing this default has no effect on:

persistent entities already in the session when the default was changed

persistent entities that are refreshed via Session.refresh(); a refreshed persistent entity will only be read-only if it wasread-only before refreshing

persistent entities added by the application via Session.persist(), Session.save(), and Session.update()Session.saveOrUpdate()

12.1.3. Loading read-only entities from an HQL query/criteria

Note

Entities of immutable classes are automatically loaded as read-only.

If Session.isDefaultReadOnly() returns false (the default) when an HQL query or criteria executes, then entities andproxies of mutable classes loaded by the query will not be read-only.

You can override this behavior so that entities and proxies loaded by an HQL query or criteria are automatically maderead-only.

For an HQL query, call:

Query.setReadOnly( true );

Query.setReadOnly( true ) must be called before Query.list(), Query.uniqueResult(), Query.scroll(), or Query.iterate()

For an HQL criteria, call:

Criteria.setReadOnly( true );

Criteria.setReadOnly( true ) must be called before Criteria.list(), Criteria.uniqueResult(), or Criteria.scroll()

Entities and proxies that exist in the session before being returned by an HQL query or criteria are not affected.

Uninitialized persistent collections returned by the query are not affected. Later, when the collection is initialized, entitiesloaded into the session will be read-only if Session.isDefaultReadOnly() returns true.

Using Query.setReadOnly( true ) or Criteria.setReadOnly( true ) works well when a single HQL query or criteria loads all the

entities and intializes all the proxies and collections that the application needs to be read-only.

When it is not possible to load and initialize all necessary entities in a single query or criteria, you can temporarily changethe session default to load entities as read-only before the query is executed. Then you can explicitly initialize proxies andcollections before restoring the session default.

Session session = factory.openSession();Transaction tx = session.beginTransaction(); setDefaultReadOnly( true );Contract contract = ( Contract ) session.createQuery( "from Contract where customerName = 'Sherman'" ) .uniqueResult();Hibernate.initialize( contract.getPlan() );Hibernate.initialize( contract.getVariations() );Hibernate.initialize( contract.getNotes() );setDefaultReadOnly( false );...tx.commit();session.close();

If Session.isDefaultReadOnly() returns true, then you can use Query.setReadOnly( false ) and Criteria.setReadOnly( false) to override this session setting and load entities that are not read-only.

12.1.4. Making a persistent entity read-only

Note

Persistent entities of immutable classes are automatically made read-only.

To make a persistent entity or proxy read-only, call:

Session.setReadOnly(entityOrProxy, true)

To change a read-only entity or proxy of a mutable class so it is no longer read-only, call:

Session.setReadOnly(entityOrProxy, false)

Important

When a read-only entity or proxy is changed so it is no longer read-only, Hibernateassumes that the current state of the read-only entity is consistent with its databaserepresentation. If this is not true, then any non-flushed changes made before or while theentity was read-only, will be ignored.

To throw away non-flushed changes and make the persistent entity consistent with its database representation, call:

session.refresh( entity );

To flush changes made before or while the entity was read-only and make the database representation consistent with thecurrent state of the persistent entity:

// evict the read-only entity so it is detachedsession.evict( entity );

// make the detached entity (with the non-flushed changes) persistentsession.update( entity );

// now entity is no longer read-only and its changes can be flusheds.flush();


The following table summarizes how different property types are affected by making an entity read-only.

Table 12.1. Affect of read-only entity on property types

Property/Association Type Changes flushed to DB?

Simple (Section 12.2.1, “Simple properties”) no*

Unidirectional one-to-one

Unidirectional many-to-one

(Section 12.2.2.1, “Unidirectional one-to-one and many-to-one”)

no*

no*

Unidirectional one-to-many

Unidirectional many-to-many

(Section 12.2.2.2, “Unidirectional one-to-many and many-to-many”)

yes

yes

Bidirectional one-to-one

(Section 12.2.3.1, “Bidirectional one-to-one”)only if the owning entity is not read-only*

Bidirectional one-to-many/many-to-one

inverse collection

non-inverse collection

(Section 12.2.3.2, “Bidirectional one-to-many/many-to-one”)

only added/removed entities that are not read-only*

yes

Bidirectional many-to-many

(Section 12.2.3.3, “Bidirectional many-to-many”)yes

Property/Association Type Changes flushed to DB?

* Behavior is different when the entity having the property/association is read-only, compared to when it is not read-only.

12.2.1. Simple properties

When a persistent object is read-only, Hibernate does not dirty-check simple properties.

Hibernate will not synchronize simple property state changes to the database. If you have automatic versioning, Hibernatewill not increment the version if any simple properties change.

Session session = factory.openSession();Transaction tx = session.beginTransaction();

// get a contract and make it read-onlyContract contract = ( Contract ) session.get( Contract.class, contractId );session.setReadOnly( contract, true );

// contract.getCustomerName() is "Sherman"contract.setCustomerName( "Yogi" );tx.commit();

tx = session.beginTransaction();

contract = ( Contract ) session.get( Contract.class, contractId );// contract.getCustomerName() is still "Sherman"...tx.commit();session.close();

12.2.2. Unidirectional associations

12.2.2.1. Unidirectional one-to-one and many-to-one

Hibernate treats unidirectional one-to-one and many-to-one associations in the same way when the owning entity is read-only.

We use the term unidirectional single-ended association when referring to functionality that is common to unidirectionalone-to-one and many-to-one associations.

Hibernate does not dirty-check unidirectional single-ended associations when the owning entity is read-only.

If you change a read-only entity's reference to a unidirectional single-ended association to null, or to refer to a differententity, that change will not be flushed to the database.

Note

If an entity is of an immutable class, then its references to unidirectional single-endedassociations must be assigned when that entity is first created. Because the entity isautomatically made read-only, these references can not be updated.

If automatic versioning is used, Hibernate will not increment the version due to local changes to unidirectional single-ended associations.

In the following examples, Contract has a unidirectional many-to-one association with Plan. Contract cascades save andupdate operations to the association.

The following shows that changing a read-only entity's many-to-one association reference to null has no effect on theentity's database representation.

// get a contract with an existing plan;// make the contract read-only and set its plan to null tx = session.beginTransaction();Contract contract = ( Contract ) session.get( Contract.class, contractId );session.setReadOnly( contract, true );contract.setPlan( null );tx.commit();

// get the same contracttx = session.beginTransaction();contract = ( Contract ) session.get( Contract.class, contractId );

// contract.getPlan() still refers to the original plan;

tx.commit();session.close();

The following shows that, even though an update to a read-only entity's many-to-one association has no affect on theentity's database representation, flush still cascades the save-update operation to the locally changed association.

// get a contract with an existing plan;// make the contract read-only and change to a new plantx = session.beginTransaction();Contract contract = ( Contract ) session.get( Contract.class, contractId );session.setReadOnly( contract, true );Plan newPlan = new Plan( "new plan"contract.setPlan( newPlan);tx.commit();

// get the same contracttx = session.beginTransaction();contract = ( Contract ) session.get( Contract.class, contractId );newPlan = ( Contract ) session.get( Plan.class, newPlan.getId() );

// contract.getPlan() still refers to the original plan;// newPlan is non-null because it was persisted when // the previous transaction was committed;

tx.commit();session.close();

12.2.2.2. Unidirectional one-to-many and many-to-many

Hibernate treats unidirectional one-to-many and many-to-many associations owned by a read-only entity the same aswhen owned by an entity that is not read-only.

Hibernate dirty-checks unidirectional one-to-many and many-to-many associations;

The collection can contain entities that are read-only, as well as entities that are not read-only.

Entities can be added and removed from the collection; changes are flushed to the database.

If automatic versioning is used, Hibernate will update the version due to changes in the collection if they dirty the owningentity.

12.2.3. Bidirectional associations

12.2.3.1. Bidirectional one-to-one

If a read-only entity owns a bidirectional one-to-one association:

Hibernate does not dirty-check the association.

updates that change the association reference to null or to refer to a different entity will not be flushed to the database.

If automatic versioning is used, Hibernate will not increment the version due to local changes to the association.

Note

If an entity is of an immutable class, and it owns a bidirectional one-to-one association,then its reference must be assigned when that entity is first created. Because the entity isautomatically made read-only, these references cannot be updated.

When the owner is not read-only, Hibernate treats an association with a read-only entity the same as when the associationis with an entity that is not read-only.

12.2.3.2. Bidirectional one-to-many/many-to-one

A read-only entity has no impact on a bidirectional one-to-many/many-to-one association if:

the read-only entity is on the one-to-many side using an inverse collection;

the read-only entity is on the one-to-many side using a non-inverse collection;

the one-to-many side uses a non-inverse collection that contains the read-only entity

When the one-to-many side uses an inverse collection:

a read-only entity can only be added to the collection when it is created;

a read-only entity can only be removed from the collection by an orphan delete or by explicitly deleting the entity.

12.2.3.3. Bidirectional many-to-many

Hibernate treats bidirectional many-to-many associations owned by a read-only entity the same as when owned by anentity that is not read-only.

Hibernate dirty-checks bidirectional many-to-many associations.

The collection on either side of the association can contain entities that are read-only, as well as entities that are not read-only.

Entities are added and removed from both sides of the collection; changes are flushed to the database.

If automatic versioning is used, Hibernate will update the version due to changes in both sides of the collection if they dirtythe entity owning the respective collections.

Chapter 13. Transactions and Concurrency

Table of Contents


13.1.1. Unit of work13.1.2. Long conversations13.1.3. Considering object identity13.1.4. Common issues


13.2.1. Non-managed environment13.2.2. Using JTA13.2.3. Exception handling13.2.4. Transaction timeout


13.3.1. Application version checking13.3.2. Extended session and automatic versioning13.3.3. Detached objects and automatic versioning13.3.4. Customizing automatic versioning

13.4. Pessimistic locking13.5. Connection release modes

The most important point about Hibernate and concurrency control is that it is easy to understand. Hibernate directly usesJDBC connections and JTA resources without adding any additional locking behavior. It is recommended that you spendsome time with the JDBC, ANSI, and transaction isolation specification of your database management system.

Hibernate does not lock objects in memory. Your application can expect the behavior as defined by the isolation level ofyour database transactions. Through Session, which is also a transaction-scoped cache, Hibernate provides repeatable

reads for lookup by identifier and entity queries and not reporting queries that return scalar values.

In addition to versioning for automatic optimistic concurrency control, Hibernate also offers, using theSELECT FOR UPDATE syntax, a (minor) API for pessimistic locking of rows. Optimistic concurrency control and this API are

discussed later in this chapter.

The discussion of concurrency control in Hibernate begins with the granularity of Configuration, SessionFactory, and Session,

as well as database transactions and long conversations.


A SessionFactory is an expensive-to-create, threadsafe object, intended to be shared by all application threads. It is created

once, usually on application startup, from a Configuration instance.

A Session is an inexpensive, non-threadsafe object that should be used once and then discarded for: a single request, a

conversation or a single unit of work. A Session will not obtain a JDBC Connection, or a Datasource, unless it is needed. It will

not consume any resources until used.

In order to reduce lock contention in the database, a database transaction has to be as short as possible. Long databasetransactions will prevent your application from scaling to a highly concurrent load. It is not recommended that you hold adatabase transaction open during user think time until the unit of work is complete.

What is the scope of a unit of work? Can a single Hibernate Session span several database transactions, or is this a one-

to-one relationship of scopes? When should you open and close a Session and how do you demarcate the database

transaction boundaries? These questions are addressed in the following sections.

13.1.1. Unit of work

First, let's define a unit of work. A unit of work is a design pattern described by Martin Fowler as “ [maintaining] a list ofobjects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrencyproblems. ”[PoEAA] In other words, its a series of operations we wish to carry out against the database together. Basically,it is a transaction, though fulfilling a unit of work will often span multiple physical database transactions (seeSection 13.1.2, “Long conversations”). So really we are talking about a more abstract notion of a transaction. The term"business transaction" is also sometimes used in lieu of unit of work.

Do not use the session-per-operation antipattern: do not open and close a Session for every simple database call in a

single thread. The same is true for database transactions. Database calls in an application are made using a plannedsequence; they are grouped into atomic units of work. This also means that auto-commit after every single SQL statementis useless in an application as this mode is intended for ad-hoc SQL console work. Hibernate disables, or expects the

application server to disable, auto-commit mode immediately. Database transactions are never optional. Allcommunication with a database has to occur inside a transaction. Auto-commit behavior for reading data should beavoided, as many small transactions are unlikely to perform better than one clearly defined unit of work. The latter is alsomore maintainable and extensible.

The most common pattern in a multi-user client/server application is session-per-request. In this model, a request from theclient is sent to the server, where the Hibernate persistence layer runs. A new Hibernate Session is opened, and all

database operations are executed in this unit of work. On completion of the work, and once the response for the client hasbeen prepared, the session is flushed and closed. Use a single database transaction to serve the clients request, startingand committing it when you open and close the Session. The relationship between the two is one-to-one and this model is

a perfect fit for many applications.

The challenge lies in the implementation. Hibernate provides built-in management of the "current session" to simplify thispattern. Start a transaction when a server request has to be processed, and end the transaction before the response issent to the client. Common solutions are ServletFilter, AOP interceptor with a pointcut on the service methods, or a

proxy/interception container. An EJB container is a standardized way to implement cross-cutting aspects such astransaction demarcation on EJB session beans, declaratively with CMT. If you use programmatic transaction demarcation,for ease of use and code portability use the Hibernate Transaction API shown later in this chapter.

Your application code can access a "current session" to process the request by calling sessionFactory.getCurrentSession().

You will always get a Session scoped to the current database transaction. This has to be configured for either resource-

local or JTA environments, see Section 2.2, “Contextual sessions”.

You can extend the scope of a Session and database transaction until the "view has been rendered". This is especially

useful in servlet applications that utilize a separate rendering phase after the request has been processed. Extending thedatabase transaction until view rendering, is achieved by implementing your own interceptor. However, this will be difficultif you rely on EJBs with container-managed transactions. A transaction will be completed when an EJB method returns,before rendering of any view can start. See the Hibernate website and forum for tips and examples relating to this OpenSession in View pattern.

13.1.2. Long conversations

The session-per-request pattern is not the only way of designing units of work. Many business processes require a wholeseries of interactions with the user that are interleaved with database accesses. In web and enterprise applications, it isnot acceptable for a database transaction to span a user interaction. Consider the following example:

The first screen of a dialog opens. The data seen by the user has been loaded in a particular Session and database

transaction. The user is free to modify the objects.

The user clicks "Save" after 5 minutes and expects their modifications to be made persistent. The user also expectsthat they were the only person editing this information and that no conflicting modification has occurred.

From the point of view of the user, we call this unit of work a long-running conversation or application transaction. Thereare many ways to implement this in your application.

A first naive implementation might keep the Session and database transaction open during user think time, with locks held

in the database to prevent concurrent modification and to guarantee isolation and atomicity. This is an anti-pattern, sincelock contention would not allow the application to scale with the number of concurrent users.

You have to use several database transactions to implement the conversation. In this case, maintaining isolation ofbusiness processes becomes the partial responsibility of the application tier. A single conversation usually spans severaldatabase transactions. It will be atomic if only one of these database transactions (the last one) stores the updated data.All others simply read data (for example, in a wizard-style dialog spanning several request/response cycles). This is easierto implement than it might sound, especially if you utilize some of Hibernate's features:

Automatic Versioning: Hibernate can perform automatic optimistic concurrency control for you. It can automaticallydetect if a concurrent modification occurred during user think time. Check for this at the end of the conversation.

Detached Objects: if you decide to use the session-per-request pattern, all loaded instances will be in the detachedstate during user think time. Hibernate allows you to reattach the objects and persist the modifications. The pattern iscalled session-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications.

Extended (or Long) Session: the Hibernate Session can be disconnected from the underlying JDBC connection after

the database transaction has been committed and reconnected when a new client request occurs. This pattern isknown as session-per-conversation and makes even reattachment unnecessary. Automatic versioning is used toisolate concurrent modifications and the Session will not be allowed to be flushed automatically, but explicitly.

Both session-per-request-with-detached-objects and session-per-conversation have advantages and disadvantages.These disadvantages are discussed later in this chapter in the context of optimistic concurrency control.

13.1.3. Considering object identity

An application can concurrently access the same persistent state in two different Sessions. However, an instance of a

persistent class is never shared between two Session instances. It is for this reason that there are two different notions of

identity:

Database Identity

foo.getId().equals( bar.getId() )

JVM Identity

foo==bar

For objects attached to a particular Session (i.e., in the scope of a Session), the two notions are equivalent and JVM identity

for database identity is guaranteed by Hibernate. While the application might concurrently access the "same" (persistentidentity) business object in two different sessions, the two instances will actually be "different" (JVM identity). Conflicts areresolved using an optimistic approach and automatic versioning at flush/commit time.

This approach leaves Hibernate and the database to worry about concurrency. It also provides the best scalability, sinceguaranteeing identity in single-threaded units of work means that it does not need expensive locking or other means ofsynchronization. The application does not need to synchronize on any business object, as long as it maintains a single

thread per Session. Within a Session the application can safely use == to compare objects.

However, an application that uses == outside of a Session might produce unexpected results. This might occur even in

some unexpected places. For example, if you put two detached instances into the same Set, both might have the same

database identity (i.e., they represent the same row). JVM identity, however, is by definition not guaranteed for instances ina detached state. The developer has to override the equals() and hashCode() methods in persistent classes and implement

their own notion of object equality. There is one caveat: never use the database identifier to implement equality. Use abusiness key that is a combination of unique, usually immutable, attributes. The database identifier will change if atransient object is made persistent. If the transient instance (usually together with detached instances) is held in a Set,

changing the hashcode breaks the contract of the Set. Attributes for business keys do not have to be as stable as database

primary keys; you only have to guarantee stability as long as the objects are in the same Set. See the Hibernate website

for a more thorough discussion of this issue. Please note that this is not a Hibernate issue, but simply how Java objectidentity and equality has to be implemented.

13.1.4. Common issues

Do not use the anti-patterns session-per-user-session or session-per-application (there are, however, rare exceptions tothis rule). Some of the following issues might also arise within the recommended patterns, so ensure that you understandthe implications before making a design decision:

A Session is not thread-safe. Things that work concurrently, like HTTP requests, session beans, or Swing workers, will

cause race conditions if a Session instance is shared. If you keep your Hibernate Session in your HttpSession (this is

discussed later in the chapter), you should consider synchronizing access to your Http session. Otherwise, a user thatclicks reload fast enough can use the same Session in two concurrently running threads.

An exception thrown by Hibernate means you have to rollback your database transaction and close the Session

immediately (this is discussed in more detail later in the chapter). If your Session is bound to the application, you have

to stop the application. Rolling back the database transaction does not put your business objects back into the statethey were at the start of the transaction. This means that the database state and the business objects will be out ofsync. Usually this is not a problem, because exceptions are not recoverable and you will have to start over afterrollback anyway.

The Session caches every object that is in a persistent state (watched and checked for dirty state by Hibernate). If you

keep it open for a long time or simply load too much data, it will grow endlessly until you get anOutOfMemoryException. One solution is to call clear() and evict() to manage the Session cache, but you should consider

a Stored Procedure if you need mass data operations. Some solutions are shown in Chapter 15, Batch processing.Keeping a Session open for the duration of a user session also means a higher probability of stale data.


Database, or system, transaction boundaries are always necessary. No communication with the database can occuroutside of a database transaction (this seems to confuse many developers who are used to the auto-commit mode).Always use clear transaction boundaries, even for read-only operations. Depending on your isolation level and databasecapabilities this might not be required, but there is no downside if you always demarcate transactions explicitly. Certainly,a single database transaction is going to perform better than many small transactions, even for reading data.

A Hibernate application can run in non-managed (i.e., standalone, simple Web- or Swing applications) and managedJ2EE environments. In a non-managed environment, Hibernate is usually responsible for its own database connectionpool. The application developer has to manually set transaction boundaries (begin, commit, or rollback databasetransactions) themselves. A managed environment usually provides container-managed transactions (CMT), with thetransaction assembly defined declaratively (in deployment descriptors of EJB session beans, for example). Programmatictransaction demarcation is then no longer necessary.

However, it is often desirable to keep your persistence layer portable between non-managed resource-localenvironments, and systems that can rely on JTA but use BMT instead of CMT. In both cases use programmatic transactiondemarcation. Hibernate offers a wrapper API called Transaction that translates into the native transaction system of your

deployment environment. This API is actually optional, but we strongly encourage its use unless you are in a CMT sessionbean.

Ending a Session usually involves four distinct phases:

flush the session

commit the transaction

close the session

handle exceptions

We discussed Flushing the session earlier, so we will now have a closer look at transaction demarcation and exceptionhandling in both managed and non-managed environments.

13.2.1. Non-managed environment

If a Hibernate persistence layer runs in a non-managed environment, database connections are usually handled bysimple (i.e., non-DataSource) connection pools from which Hibernate obtains connections as needed. Thesession/transaction handling idiom looks like this:

// Non-managed environment idiomSession sess = factory.openSession();Transaction tx = null;try { tx = sess.beginTransaction();

// do some work ...

tx.commit();}catch (RuntimeException e) { if (tx != null) tx.rollback(); throw e; // or display error message}finally { sess.close();}

You do not have to flush() the Session explicitly: the call to commit() automatically triggers the synchronization depending on

the Section 11.10, “Flushing the Session” for the session. A call to close() marks the end of a session. The main implication

of close() is that the JDBC connection will be relinquished by the session. This Java code is portable and runs in both non-

managed and JTA environments.

As outlined earlier, a much more flexible solution is Hibernate's built-in "current session" context management:

// Non-managed environment idiom with getCurrentSession()try { factory.getCurrentSession().beginTransaction();

// do some work ...

factory.getCurrentSession().getTransaction().commit();}catch (RuntimeException e) { factory.getCurrentSession().getTransaction().rollback(); throw e; // or display error message}

You will not see these code snippets in a regular application; fatal (system) exceptions should always be caught at the"top". In other words, the code that executes Hibernate calls in the persistence layer, and the code that handlesRuntimeException (and usually can only clean up and exit), are in different layers. The current context management by

Hibernate can significantly simplify this design by accessing a SessionFactory. Exception handling is discussed later in this

chapter.

You should select org.hibernate.transaction.JDBCTransactionFactory, which is the default, and for the second example select

"thread" as your hibernate.current_session_context_class.

13.2.2. Using JTA

If your persistence layer runs in an application server (for example, behind EJB session beans), every datasourceconnection obtained by Hibernate will automatically be part of the global JTA transaction. You can also install astandalone JTA implementation and use it without EJB. Hibernate offers two strategies for JTA integration.

If you use bean-managed transactions (BMT), Hibernate will tell the application server to start and end a BMT transactionif you use the Transaction API. The transaction management code is identical to the non-managed environment.

// BMT idiomSession sess = factory.openSession();Transaction tx = null;try { tx = sess.beginTransaction();

// do some work ...

tx.commit();}catch (RuntimeException e) { if (tx != null) tx.rollback(); throw e; // or display error message}finally { sess.close();}

If you want to use a transaction-bound Session, that is, the getCurrentSession() functionality for easy context propagation,

use the JTA UserTransaction API directly:

// BMT idiom with getCurrentSession()try { UserTransaction tx = (UserTransaction)new InitialContext() .lookup("java:comp/UserTransaction");

tx.begin();

// Do some work on Session bound to transaction factory.getCurrentSession().load(...); factory.getCurrentSession().persist(...);

tx.commit();}catch (RuntimeException e) { tx.rollback(); throw e; // or display error message}

With CMT, transaction demarcation is completed in session bean deployment descriptors, not programmatically. The codeis reduced to:

// CMT idiom Session sess = factory.getCurrentSession();

// do some work ...

In a CMT/EJB, even rollback happens automatically. An unhandled RuntimeException thrown by a session bean method

tells the container to set the global transaction to rollback. You do not need to use the Hibernate Transaction API at all with

BMT or CMT, and you get automatic propagation of the "current" Session bound to the transaction.

When configuring Hibernate's transaction factory, choose org.hibernate.transaction.JTATransactionFactory if you use JTA

directly (BMT), and org.hibernate.transaction.CMTTransactionFactory in a CMT session bean. Remember to also set

hibernate.transaction.manager_lookup_class. Ensure that your hibernate.current_session_context_class is either unset

(backwards compatibility), or is set to "jta".

The getCurrentSession() operation has one downside in a JTA environment. There is one caveat to the use of after_statement

connection release mode, which is then used by default. Due to a limitation of the JTA spec, it is not possible for Hibernateto automatically clean up any unclosed ScrollableResults or Iterator instances returned by scroll() or iterate(). You must

release the underlying database cursor by calling ScrollableResults.close() or Hibernate.close(Iterator) explicitly from a finally

block. Most applications can easily avoid using scroll() or iterate() from the JTA or CMT code.)

13.2.3. Exception handling

If the Session throws an exception, including any SQLException, immediately rollback the database transaction, call

Session.close() and discard the Session instance. Certain methods of Session will not leave the session in a consistent state.

No exception thrown by Hibernate can be treated as recoverable. Ensure that the Session will be closed by calling close()

in a finally block.

The HibernateException, which wraps most of the errors that can occur in a Hibernate persistence layer, is an unchecked

exception. It was not in older versions of Hibernate. In our opinion, we should not force the application developer to catchan unrecoverable exception at a low layer. In most systems, unchecked and fatal exceptions are handled in one of the firstframes of the method call stack (i.e., in higher layers) and either an error message is presented to the application user orsome other appropriate action is taken. Note that Hibernate might also throw other unchecked exceptions that are not aHibernateException. These are not recoverable and appropriate action should be taken.

Hibernate wraps SQLExceptions thrown while interacting with the database in a JDBCException. In fact, Hibernate will

attempt to convert the exception into a more meaningful subclass of JDBCException. The underlying SQLException is always

available via JDBCException.getCause(). Hibernate converts the SQLException into an appropriate JDBCException subclass

using the SQLExceptionConverter attached to the SessionFactory. By default, the SQLExceptionConverter is defined by the

configured dialect. However, it is also possible to plug in a custom implementation. See the javadocs for theSQLExceptionConverterFactory class for details. The standard JDBCException subtypes are:

JDBCConnectionException: indicates an error with the underlying JDBC communication.

SQLGrammarException: indicates a grammar or syntax problem with the issued SQL.

ConstraintViolationException: indicates some form of integrity constraint violation.

LockAcquisitionException: indicates an error acquiring a lock level necessary to perform the requested operation.

GenericJDBCException: a generic exception which did not fall into any of the other categories.

13.2.4. Transaction timeout

An important feature provided by a managed environment like EJB, that is never provided for non-managed code, istransaction timeout. Transaction timeouts ensure that no misbehaving transaction can indefinitely tie up resources whilereturning no response to the user. Outside a managed (JTA) environment, Hibernate cannot fully provide this functionality.However, Hibernate can at least control data access operations, ensuring that database level deadlocks and queries withhuge result sets are limited by a defined timeout. In a managed environment, Hibernate can delegate transaction timeoutto JTA. This functionality is abstracted by the Hibernate Transaction object.

Session sess = factory.openSession();try { //set transaction timeout to 3 seconds sess.getTransaction().setTimeout(3); sess.getTransaction().begin();

// do some work ...

sess.getTransaction().commit()}catch (RuntimeException e) { sess.getTransaction().rollback(); throw e; // or display error message}finally { sess.close();}

setTimeout() cannot be called in a CMT bean, where transaction timeouts must be defined declaratively.


The only approach that is consistent with high concurrency and high scalability, is optimistic concurrency control withversioning. Version checking uses version numbers, or timestamps, to detect conflicting updates and to prevent lostupdates. Hibernate provides three possible approaches to writing application code that uses optimistic concurrency. Theuse cases we discuss are in the context of long conversations, but version checking also has the benefit of preventing lostupdates in single database transactions.

13.3.1. Application version checking

In an implementation without much help from Hibernate, each interaction with the database occurs in a new Session and

the developer is responsible for reloading all persistent instances from the database before manipulating them. The

application is forced to carry out its own version checking to ensure conversation transaction isolation. This approach isthe least efficient in terms of database access. It is the approach most similar to entity EJBs.

// foo is an instance loaded by a previous Sessionsession = factory.openSession();Transaction t = session.beginTransaction();

int oldVersion = foo.getVersion();session.load( foo, foo.getKey() ); // load the current stateif ( oldVersion != foo.getVersion() ) throw new StaleObjectStateException();foo.setProperty("bar");

t.commit();session.close();

The version property is mapped using <version>, and Hibernate will automatically increment it during flush if the entity is

dirty.

If you are operating in a low-data-concurrency environment, and do not require version checking, you can use thisapproach and skip the version check. In this case, last commit wins is the default strategy for long conversations. Be awarethat this might confuse the users of the application, as they might experience lost updates without error messages or achance to merge conflicting changes.

Manual version checking is only feasible in trivial circumstances and not practical for most applications. Often not onlysingle instances, but complete graphs of modified objects, have to be checked. Hibernate offers automatic versionchecking with either an extended Session or detached instances as the design paradigm.

13.3.2. Extended session and automatic versioning

A single Session instance and its persistent instances that are used for the whole conversation are known as session-per-

conversation. Hibernate checks instance versions at flush time, throwing an exception if concurrent modification isdetected. It is up to the developer to catch and handle this exception. Common options are the opportunity for the user tomerge changes or to restart the business conversation with non-stale data.

The Session is disconnected from any underlying JDBC connection when waiting for user interaction. This approach is the

most efficient in terms of database access. The application does not version check or reattach detached instances, nordoes it have to reload instances in every database transaction.

// foo is an instance loaded earlier by the old sessionTransaction t = session.beginTransaction(); // Obtain a new JDBC connection, start transaction

foo.setProperty("bar");

session.flush(); // Only for last transaction in conversationt.commit(); // Also return JDBC connectionsession.close(); // Only for last transaction in conversation

The foo object knows which Session it was loaded in. Beginning a new database transaction on an old session obtains a

new connection and resumes the session. Committing a database transaction disconnects a session from the JDBCconnection and returns the connection to the pool. After reconnection, to force a version check on data you are notupdating, you can call Session.lock() w i th LockMode.READ on any objects that might have been updated by another

transaction. You do not need to lock any data that you are updating. Usually you would set FlushMode.MANUAL on an

extended Session, so that only the last database transaction cycle is allowed to actually persist all modifications made in

this conversation. Only this last database transaction will include the flush() operation, and then close() the session to end

the conversation.

This pattern is problematic if the Session is too big to be stored during user think time (for example, an HttpSession should

be kept as small as possible). As the Session is also the first-level cache and contains all loaded objects, we can probably

use this strategy only for a few request/response cycles. Use a Session only for a single conversation as it will soon have

stale data.

Note

Earlier versions of Hibernate required explicit disconnection and reconnection of aSession. These methods are deprecated, as beginning and ending a transaction has the

same effect.

Keep the disconnected Session close to the persistence layer. Use an EJB stateful session bean to hold the Session in a

three-tier environment. Do not transfer it to the web layer, or even serialize it to a separate tier, to store it in the HttpSession.

The extended session pattern, or session-per-conversation, is more difficult to implement with automatic current sessioncontext management. You need to supply your own implementation of the CurrentSessionContext for this. See the Hibernate

Wiki for examples.

13.3.3. Detached objects and automatic versioning

Each interaction with the persistent store occurs in a new Session. However, the same persistent instances are reused for

each interaction with the database. The application manipulates the state of detached instances originally loaded inanother Session and then reattaches them using Session.update(), Session.saveOrUpdate(), or Session.merge().

// foo is an instance loaded by a previous Sessionfoo.setProperty("bar");session = factory.openSession();Transaction t = session.beginTransaction();session.saveOrUpdate(foo); // Use merge() if "foo" might have been loaded alreadyt.commit();session.close();

Again, Hibernate will check instance versions during flush, throwing an exception if conflicting updates occurred.

You can also call lock() instead of update(), and use LockMode.READ (performing a version check and bypassing all caches)

if you are sure that the object has not been modified.

13.3.4. Customizing automatic versioning

You can disable Hibernate's automatic version increment for particular properties and collections by setting theoptimistic-lock mapping attribute to false. Hibernate will then no longer increment versions if the property is dirty.

Legacy database schemas are often static and cannot be modified. Or, other applications might access the samedatabase and will not know how to handle version numbers or even timestamps. In both cases, versioning cannot rely ona particular column in a table. To force a version check with a comparison of the state of all fields in a row but without aversion or timestamp property mapping, turn on optimistic-lock="all" in the <class> mapping. This conceptually only works if

Hibernate can compare the old and the new state (i.e., if you use a single long Session and not session-per-request-with-

detached-objects).

Concurrent modification can be permitted in instances where the changes that have been made do not overlap. If you setoptimistic-lock="dirty" when mapping the <class>, Hibernate will only compare dirty fields during flush.

In both cases, with dedicated version/timestamp columns or with a full/dirty field comparison, Hibernate uses a singleUPDATE statement, with an appropriate WHERE clause, per entity to execute the version check and update the information.

If you use transitive persistence to cascade reattachment to associated entities, Hibernate may execute unnecessaryupdates. This is usually not a problem, but on update triggers in the database might be executed even when no changeshave been made to detached instances. You can customize this behavior by setting select-before-update="true" in the

<class> mapping, forcing Hibernate to SELECT the instance to ensure that changes did occur before updating the row.

13.4. Pessimistic locking

It is not intended that users spend much time worrying about locking strategies. It is usually enough to specify an isolationlevel for the JDBC connections and then simply let the database do all the work. However, advanced users may wish toobtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction.

Hibernate will always use the locking mechanism of the database; it never lock objects in memory.

The LockMode class defines the different lock levels that can be acquired by Hibernate. A lock is obtained by the following

mechanisms:

LockMode.WRITE is acquired automatically when Hibernate updates or inserts a row.

LockMode.UPGRADE can be acquired upon explicit user request using SELECT ... FOR UPDATE on databases which

support that syntax.

LockMode.UPGRADE_NOWAIT can be acquired upon explicit user request using a SELECT ... FOR UPDATE NOWAIT

under Oracle.

LockMode.READ is acquired automatically when Hibernate reads data under Repeatable Read or Serializable isolation

level. It can be re-acquired by explicit user request.

LockMode.NONE represents the absence of a lock. All objects switch to this lock mode at the end of a Transaction.

Objects associated with the session via a call to update() or saveOrUpdate() also start out in this lock mode.

The "explicit user request" is expressed in one of the following ways:

A call to Session.load(), specifying a LockMode.

A call to Session.lock().

A call to Query.setLockMode().

If Session.load() is called with UPGRADE or UPGRADE_NOWAIT, and the requested object was not yet loaded by the session,

the object is loaded using SELECT ... FOR UPDATE. I f load() is called for an object that is already loaded with a less

restrictive lock than the one requested, Hibernate calls lock() for that object.

Session.lock() performs a version number check if the specified lock mode is READ, UPGRADE or UPGRADE_NOWAIT. In the

case of UPGRADE or UPGRADE_NOWAIT, SELECT ... FOR UPDATE is used.

If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead ofthrowing an exception. This ensures that applications are portable.

13.5. Connection release modes

One of the legacies of Hibernate 2.x JDBC connection management meant that a Session would obtain a connection when

it was first required and then maintain that connection until the session was closed. Hibernate 3.x introduced the notion ofconnection release modes that would instruct a session how to handle its JDBC connections. The following discussion ispertinent only to connections provided through a configured ConnectionProvider. User-supplied connections are outside the

breadth of this discussion. The different release modes are identified by the enumerated values oforg.hibernate.ConnectionReleaseMode:

ON_CLOSE: is the legacy behavior described above. The Hibernate session obtains a connection when it first needs to

perform some JDBC access and maintains that connection until the session is closed.

AFTER_TRANSACTION: releases connections after a org.hibernate.Transaction has been completed.

AFTER_STATEMENT (also referred to as aggressive release): releases connections after every statement execution.

This aggressive releasing is skipped if that statement leaves open resources associated with the given session.Currently the only situation where this occurs is through the use of org.hibernate.ScrollableResults.

The configuration parameter hibernate.connection.release_mode is used to specify which release mode to use. The possible

values are as follows:

auto (the default): this choice delegates to the release mode returned by the

org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode() method. For JTATransactionFactory, this returns

ConnectionReleaseMode.AFTER_STATEMENT; for JDBCTransactionFactory, this returns

ConnectionReleaseMode.AFTER_TRANSACTION. Do not change this default behavior as failures due to the value ofthis setting tend to indicate bugs and/or invalid assumptions in user code.

on_close: uses ConnectionReleaseMode.ON_CLOSE. This setting is left for backwards compatibility, but its use is

discouraged.

after_transaction: uses ConnectionReleaseMode.AFTER_TRANSACTION. This setting should not be used in JTA

environments. Also note that with ConnectionReleaseMode.AFTER_TRANSACTION, if a session is considered to bein auto-commit mode, connections will be released as if the release mode were AFTER_STATEMENT.

after_statement: uses ConnectionReleaseMode.AFTER_STATEMENT. Additionally, the configured ConnectionProvider is

consulted to see if it supports this setting (supportsAggressiveRelease()) . If not, the release mode is reset to

ConnectionReleaseMode.AFTER_TRANSACTION. This setting is only safe in environments where we can either re-acquire the same underlying JDBC connection each time you make a call into ConnectionProvider.getConnection() or in

auto-commit environments where it does not matter if we re-establish the same connection.

Chapter 14. Interceptors and events

Table of Contents

14.1. Interceptors14.2. Event system14.3. Hibernate declarative security

It is useful for the application to react to certain events that occur inside Hibernate. This allows for the implementation ofgeneric functionality and the extension of Hibernate functionality.

14.1. Interceptors

The Interceptor interface provides callbacks from the session to the application, allowing the application to inspect and/or

manipulate properties of a persistent object before it is saved, updated, deleted or loaded. One possible use for this is totrack auditing information. For example, the following Interceptor automatically sets the createTimestamp when an Auditable is

created and updates the lastUpdateTimestamp property when an Auditable is updated.

You can either implement Interceptor directly or extend EmptyInterceptor.

package org.hibernate.test;

import java.io.Serializable;import java.util.Date;import java.util.Iterator;

import org.hibernate.EmptyInterceptor;import org.hibernate.Transaction;import org.hibernate.type.Type;

public class AuditInterceptor extends EmptyInterceptor {

private int updates; private int creates; private int loads;

public void onDelete(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { // do nothing }

public boolean onFlushDirty(Object entity, Serializable id, Object[] currentState, Object[] previousState, String[] propertyNames, Type[] types) {

if ( entity instanceof Auditable ) { updates++; for ( int i=0; i < propertyNames.length; i++ ) { if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) { currentState[i] = new Date(); return true; } } } return false; }

public boolean onLoad(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { if ( entity instanceof Auditable ) { loads++; } return false; }

public boolean onSave(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) {

if ( entity instanceof Auditable ) { creates++; for ( int i=0; i<propertyNames.length; i++ ) { if ( "createTimestamp".equals( propertyNames[i] ) ) { state[i] = new Date(); return true; } } } return false; }

public void afterTransactionCompletion(Transaction tx) { if ( tx.wasCommitted() ) { System.out.println("Creations: " + creates + ", Updates: " + updates + "Loads: " + loads); } updates=0; creates=0; loads=0; }

}

There are two kinds of inteceptors: Session-scoped and SessionFactory-scoped.

A Session-scoped interceptor is specified when a session is opened.

Session session = sf.withOptions( new AuditInterceptor() ).openSession();

A SessionFactory-scoped interceptor is registered with the Configuration object prior to building the SessionFactory. Unless a

session is opened explicitly specifying the interceptor to use, the supplied interceptor will be applied to all sessionsopened from that SessionFactory. SessionFactory-scoped interceptors must be thread safe. Ensure that you do not store

session-specific states, since multiple sessions will use this interceptor potentially concurrently.

new Configuration().setInterceptor( new AuditInterceptor() );

14.2. Event system

If you have to react to particular events in your persistence layer, you can also use the Hibernate event architecture. Theevent system can be used in addition, or as a replacement, for interceptors.

Many methods of the Session interface correlate to an event type. The full range of defined event types is declared as

enum values on org.hibernate.event.spi.EventType. When a request is made of one of these methods, the Hibernate Session

generates an appropriate event and passes it to the configured event listeners for that type. Out-of-the-box, these listenersimplement the same processing in which those methods always resulted. However, you are free to implement acustomization of one of the listener interfaces (i.e., the LoadEvent is processed by the registered implementation of the

LoadEventListener interface), in which case their implementation would be responsible for processing any load() requests

made of the Session.

Note

See the Hibernate Developer Guide for information on registering custom eventlisteners.

The listeners should be considered stateless; they are shared between requests, and should not save any state asinstance variables.

A custom listener implements the appropriate interface for the event it wants to process and/or extend one of theconvenience base classes (or even the default event listeners used by Hibernate out-of-the-box as these are declarednon-final for this purpose). Here is an example of a custom load event listener:

public class MyLoadListener implements LoadEventListener { // this is the single method defined by the LoadEventListener interface public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType) throws HibernateException { if ( !MySecurity.isAuthorized( event.getEntityClassName(), event.getEntityId() ) ) { throw MySecurityException("Unauthorized access"); } }}

14.3. Hibernate declarative security

Usually, declarative security in Hibernate applications is managed in a session facade layer. Hibernate allows certainactions to be permissioned via JACC, and authorized via JAAS. This is an optional functionality that is built on top of theevent architecture.

First, you must configure the appropriate event listeners, to enable the use of JACC authorization. Again, see HibernateDeveloper Guide for the details. Below is an example of an appropriate org.hibernate.integrator.spi.Integrator implementation

for this purpose.

import org.hibernate.event.service.spi.DuplicationStrategy;import org.hibernate.event.service.spi.EventListenerRegistry;import org.hibernate.integrator.spi.Integrator;import org.hibernate.secure.internal.JACCPreDeleteEventListener;import org.hibernate.secure.internal.JACCPreInsertEventListener;import org.hibernate.secure.internal.JACCPreLoadEventListener;import org.hibernate.secure.internal.JACCPreUpdateEventListener;import org.hibernate.secure.internal.JACCSecurityListener;

public class JaccEventListenerIntegrator implements Integrator {

private static final DuplicationStrategy JACC_DUPLICATION_STRATEGY = new DuplicationStrategy() { @Override public boolean areMatch(Object listener, Object original) { return listener.getClass().equals( original.getClass() ) && JACCSecurityListener.class.isInstance( original ); }

@Override public Action getAction() { return Action.KEEP_ORIGINAL; } };

@Override @SuppressWarnings( {"unchecked"}) public void integrate( Configuration configuration, SessionFactoryImplementor sessionFactory, SessionFactoryServiceRegistry serviceRegistry) { boolean isSecurityEnabled = configuration.getProperties().containsKey( AvailableSettings.JACC_ENABLED ); if ( !isSecurityEnabled ) { return; }

final EventListenerRegistry eventListenerRegistry = serviceRegistry.getService( EventListenerRegistry.class ); eventListenerRegistry.addDuplicationStrategy( JACC_DUPLICATION_STRATEGY );

final String jaccContextId = configuration.getProperty( Environment.JACC_CONTEXTID ); eventListenerRegistry.prependListeners( EventType.PRE_DELETE, new JACCPreDeleteEventListener(jaccContextId) ); eventListenerRegistry.prependListeners( EventType.PRE_INSERT, new JACCPreInsertEventListener(jaccContextId) ); eventListenerRegistry.prependListeners( EventType.PRE_UPDATE, new JACCPreUpdateEventListener(jaccContextId) ); eventListenerRegistry.prependListeners( EventType.PRE_LOAD, new JACCPreLoadEventListener(jaccContextId) ); }}

You must also decide how to configure your JACC provider. One option is to tell Hibernate what permissions to bind towhat roles and have it configure the JACC provider. This would be done in the hibernate.cfg.xml file.

<grant role="admin" entity-name="User" actions="insert,update,read"/><grant role="su" entity-name="User" actions="*"/>

Chapter 15. Batch processing

Table of Contents

15.1. Batch inserts15.2. Batch updates15.3. The StatelessSession interface15.4. DML-style operations

A naive approach to inserting 100,000 rows in the database using Hibernate might look like this:

Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction();for ( int i=0; i<100000; i++ ) { Customer customer = new Customer(.....); session.save(customer);}tx.commit();session.close();

This would fall over with an OutOfMemoryException somewhere around the 50,000th row. That is because Hibernate

caches all the newly inserted Customer instances in the session-level cache. In this chapter we will show you how to avoid

this problem.

If you are undertaking batch processing you will need to enable the use of JDBC batching. This is absolutely essential ifyou want to achieve optimal performance. Set the JDBC batch size to a reasonable number (10-50, for example):

hibernate.jdbc.batch_size 20

Hibernate disables insert batching at the JDBC level transparently if you use an identity identifier generator.

You can also do this kind of work in a process where interaction with the second-level cache is completely disabled:

hibernate.cache.use_second_level_cache false

However, this is not absolutely necessary, since we can explicitly set the CacheMode to disable interaction with the second-

level cache.

15.1. Batch inserts

When making new objects persistent flush() and then clear() the session regularly in order to control the size of the first-

level cache.

Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction(); for ( int i=0; i<100000; i++ ) { Customer customer = new Customer(.....); session.save(customer); if ( i % 20 == 0 ) { //20, same as the JDBC batch size //flush a batch of inserts and release memory: session.flush(); session.clear(); }} tx.commit();session.close();

15.2. Batch updates

For retrieving and updating data, the same ideas apply. In addition, you need to use scroll() to take advantage of server-

side cursors for queries that return many rows of data.

Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction(); ScrollableResults customers = session.getNamedQuery("GetCustomers") .setCacheMode(CacheMode.IGNORE) .scroll(ScrollMode.FORWARD_ONLY);int count=0;while ( customers.next() ) { Customer customer = (Customer) customers.get(0); customer.updateStuff(...); if ( ++count % 20 == 0 ) { //flush a batch of updates and release memory: session.flush(); session.clear(); }} tx.commit();session.close();

15.3. The StatelessSession interface

Alternatively, Hibernate provides a command-oriented API that can be used for streaming data to and from the database inthe form of detached objects. A StatelessSession has no persistence context associated with it and does not provide many

of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interactwith any second-level or query cache. It does not implement transactional write-behind or automatic dirty checking.Operations performed using a stateless session never cascade to associated instances. Collections are ignored by astateless session. Operations performed via a stateless session bypass Hibernate's event model and interceptors. Due tothe lack of a first-level cache, Stateless sessions are vulnerable to data aliasing effects. A stateless session is a lower-level abstraction that is much closer to the underlying JDBC.

StatelessSession session = sessionFactory.openStatelessSession();Transaction tx = session.beginTransaction(); ScrollableResults customers = session.getNamedQuery("GetCustomers") .scroll(ScrollMode.FORWARD_ONLY);while ( customers.next() ) { Customer customer = (Customer) customers.get(0); customer.updateStuff(...); session.update(customer);} tx.commit();session.close();

In this code example, the Customer instances returned by the query are immediately detached. They are never associated

with any persistence context.

The insert(), update() and delete() operations defined by the StatelessSession interface are considered to be direct database

row-level operations. They result in the immediate execution of a SQL INSERT, UPDATE or DELETE respectively. They have

different semantics to the save(), saveOrUpdate() and delete() operations defined by the Session interface.

15.4. DML-style operations

As already discussed, automatic and transparent object/relational mapping is concerned with the management of theobject state. The object state is available in memory. This means that manipulating data directly in the database (using theSQL Data Manipulation Language (DML) the statements: INSERT, UPDATE, DELETE) will not affect in-memory state. However,

Hibernate provides methods for bulk SQL-style DML statement execution that is performed through the Hibernate QueryLanguage (Chapter 16, HQL: The Hibernate Query Language).

The pseudo-syntax for UPDATE and DELETE statements is:

( UPDATE | DELETE ) FROM? EntityName (WHERE where_conditions)?.

Some points to note:

In the from-clause, the FROM keyword is optional

There can only be a single entity named in the from-clause. It can, however, be aliased. If the entity name is aliased,then any property references must be qualified using that alias. If the entity name is not aliased, then it is illegal for anyproperty references to be qualified.

No Section 16.4, “Forms of join syntax”, either implicit or explicit, can be specified in a bulk HQL query. Sub-queriescan be used in the where-clause, where the subqueries themselves may contain joins.

The where-clause is also optional.

As an example, to execute an HQL UPDATE, use the Query.executeUpdate() method. The method is named for those

familiar with JDBC's PreparedStatement.executeUpdate():

Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction();

String hqlUpdate = "update Customer c set c.name = :newName where c.name = :oldName";// or String hqlUpdate = "update Customer set name = :newName where name = :oldName";int updatedEntities = s.createQuery( hqlUpdate ) .setString( "newName", newName ) .setString( "oldName", oldName ) .executeUpdate();tx.commit();session.close();

In keeping with the EJB3 specification, HQL UPDATE statements, by default, do not effect the Section 5.1.3.1, “Version

number” or the Section 5.1.3.2, “Timestamp” property values for the affected entities. However, you can force Hibernate toreset the version or timestamp property values through the use of a versioned update. This is achieved by adding the

VERSIONED keyword after the UPDATE keyword.

Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction();String hqlVersionedUpdate = "update versioned Customer set name = :newName where name = :oldName";int updatedEntities = s.createQuery( hqlUpdate ) .setString( "newName", newName ) .setString( "oldName", oldName ) .executeUpdate();tx.commit();session.close();

Custom version types, org.hibernate.usertype.UserVersionType, are not allowed in conjunction with a update versioned

statement.

To execute an HQL DELETE, use the same Query.executeUpdate() method:


String hqlDelete = "delete Customer c where c.name = :oldName";// or String hqlDelete = "delete Customer where name = :oldName";int deletedEntities = s.createQuery( hqlDelete ) .setString( "oldName", oldName ) .executeUpdate();tx.commit();session.close();

The int value returned by the Query.executeUpdate() method indicates the number of entities effected by the operation. This

may or may not correlate to the number of rows effected in the database. An HQL bulk operation might result in multipleactual SQL statements being executed (for joined-subclass, for example). The returned number indicates the number ofactual entities affected by the statement. Going back to the example of joined-subclass, a delete against one of thesubclasses may actually result in deletes against not just the table to which that subclass is mapped, but also the "root"table and potentially joined-subclass tables further down the inheritance hierarchy.

The pseudo-syntax for INSERT statements is: INSERT INTO EntityName properties_list select_statement. Some points to note:

Only the INSERT INTO ... SELECT ... form is supported; not the INSERT INTO ... VALUES ... form.

The properties_list is analogous to the column specification in the SQL INSERT statement. For entities involved in

mapped inheritance, only properties directly defined on that given class-level can be used in the properties_list.Superclass properties are not allowed and subclass properties do not make sense. In other words, INSERT statements

are inherently non-polymorphic.

select_statement can be any valid HQL select query, with the caveat that the return types must match the typesexpected by the insert. Currently, this is checked during query compilation rather than allowing the check to relegate tothe database. This might, however, cause problems between Hibernate Types which are equivalent as opposed to

equal. This might cause issues with mismatches between a property defined as a org.hibernate.type.DateType and a

property defined as a org.hibernate.type.TimestampType, even though the database might not make a distinction or might

be able to handle the conversion.

For the id property, the insert statement gives you two options. You can either explicitly specify the id property in theproperties_list, in which case its value is taken from the corresponding select expression, or omit it from theproperties_list, in which case a generated value is used. This latter option is only available when using id generatorsthat operate in the database; attempting to use this option with any "in memory" type generators will cause anexception during parsing. For the purposes of this discussion, in-database generators are considered to beorg.hibernate.id.SequenceGenerator (and its subclasses) and any implementers of

org.hibernate.id.PostInsertIdentifierGenerator. The most notable exception here is org.hibernate.id.TableHiLoGenerator, which

cannot be used because it does not expose a selectable way to get its values.

For properties mapped as either version or timestamp, the insert statement gives you two options. You can either specify

the property in the properties_list, in which case its value is taken from the corresponding select expressions, or omit itfrom the properties_list, in which case the seed value defined by the org.hibernate.type.VersionType is used.

The following is an example of an HQL INSERT statement execution:


String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ...";int createdEntities = s.createQuery( hqlInsert ) .executeUpdate();tx.commit();session.close();

Chapter 16. HQL: The Hibernate Query Language

Table of Contents

16.1. Case Sensitivity16.2. The from clause16.3. Associations and joins16.4. Forms of join syntax16.5. Referring to identifier property16.6. The select clause16.7. Aggregate functions16.8. Polymorphic queries16.9. The where clause16.10. Expressions16.11. The order by clause16.12. The group by clause16.13. Subqueries16.14. HQL examples16.15. Bulk update and delete16.16. Tips & Tricks16.17. Components16.18. Row value constructor syntax

Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with SQL, however,HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

16.1. Case Sensitivity

With the exception of names of Java classes and properties, queries are case-insensitive. S o SeLeCT is the same as

sELEct is the same as SELECT, but org.hibernate.eg.FOO is not org.hibernate.eg.Foo, and foo.barSet is not foo.BARSET.

This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but thisconvention is unsuitable for queries embedded in Java code.

16.2. The from clause

The simplest possible Hibernate query is of the form:

from eg.Cat

This returns all instances of the class eg.Cat. You do not usually need to qualify the class name, since auto-import is the

default. For example:

from Cat

In order to refer to the Cat in other parts of the query, you will need to assign an alias. For example:

from Cat as cat

This query assigns the alias cat to Cat instances, so you can use that alias later in the query. The as keyword is optional.

You could also write:

from Cat cat

Multiple classes can appear, resulting in a cartesian product or "cross" join.

from Formula, Parameter

from Formula as form, Parameter as param

It is good practice to name query aliases using an initial lowercase as this is consistent with Java naming standards forlocal variables (e.g. domesticCat).

16.3. Associations and joins

You can also assign aliases to associated entities or to elements of a collection of values using a join. For example:

from Cat as cat inner join cat.mate as mate left outer join cat.kittens as kitten

from Cat as cat left join cat.mate.kittens as kittens

from Formula form full join form.parameter param

The supported join types are borrowed from ANSI SQL:

inner join

left outer join

right outer join

full join (not usually useful)

The inner join, left outer join and right outer join constructs may be abbreviated.

from Cat as cat join cat.mate as mate left join cat.kittens as kitten

You may supply extra join conditions using the HQL with keyword.

from Cat as cat left join cat.kittens as kitten with kitten.bodyWeight > 10.0

A "fetch" join allows associations or collections of values to be initialized along with their parent objects using a singleselect. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations ofthe mapping file for associations and collections. See Section 20.1, “Fetching strategies” for more information.

from Cat as cat inner join fetch cat.mate left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in the where

clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they maybe accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a furthercollection:

from Cat as cat inner join fetch cat.mate left join fetch cat.kittens child left join fetch child.kittens

The fetch construct cannot be used in queries called using iterate() (though scroll() can be used). Fetch should not be used

together with setMaxResults() or setFirstResult(), as these operations are based on the result rows which usually contain

duplicates for eager collection fetching, hence, the number of rows is not what you would expect. Fetch should also not be

used together with impromptu with condition. It is possible to create a cartesian product by join fetching more than one

collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results forbag mappings, so user discretion is advised when formulating queries in this case. Finally, note that full join fetch and

right join fetch are not meaningful.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch thelazy properties in the first query immediately using fetch all properties.

from Document fetch all properties order by name

from Document doc fetch all properties where lower(doc.name) like '%cats%'

16.4. Forms of join syntax

HQL supports two forms of association joining: implicit and explicit.

The queries shown in the previous section all use the explicit form, that is, where the join keyword is explicitly used in the

from clause. This is the recommended form.

The implicit form does not use the join keyword. Instead, the associations are "dereferenced" using dot-notation. implicit

joins can appear in any of the HQL clauses. implicit join result in inner joins in the resulting SQL statement.

from Cat as cat where cat.mate.name like '%s%'

16.5. Referring to identifier property

There are 2 ways to refer to an entity's identifier property:

The special property (lowercase) id may be used to reference the identifier property of an entity provided that the entity

does not define a non-identifier property named id.

If the entity defines a named identifier property, you can use that property name.

References to composite identifier properties follow the same naming rules. If the entity has a non-identifier propertynamed id, the composite identifier property can only be referenced by its defined named. Otherwise, the special id property

can be used to reference the identifier property.

Important

Please note that, starting in version 3.2.2, this has changed significantly. In previousversions, id always referred to the identifier property regardless of its actual name. A

ramification of that decision was that non-identifier properties named id could never be

referenced in Hibernate queries.

16.6. The select clause

The select clause picks which objects and properties to return in the query result set. Consider the following:

select matefrom Cat as cat inner join cat.mate as mate

The query will select mates of other Cats. You can express this query more compactly as:

select cat.mate from Cat cat

Queries can return properties of any value type including properties of component type:

select cat.name from DomesticCat catwhere cat.name like 'fri%'

select cust.name.firstName from Customer as cust

Queries can return multiple objects and/or properties as an array of type Object[]:

select mother, offspr, mate.namefrom DomesticCat as mother inner join mother.mate as mate left outer join mother.kittens as offspr

Or as a List:

select new list(mother, offspr, mate.name)from DomesticCat as mother inner join mother.mate as mate left outer join mother.kittens as offspr

Or - assuming that the class Family has an appropriate constructor - as an actual typesafe Java object:

select new Family(mother, mate, offspr)from DomesticCat as mother join mother.mate as mate left join mother.kittens as offspr

You can assign aliases to selected expressions using as:

select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as nfrom Cat cat

This is most useful when used together with select new map:

select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )from Cat cat

This query returns a Map from aliases to selected values.

16.7. Aggregate functions

HQL queries can even return the results of aggregate functions on properties:

select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)from Cat cat

The supported aggregate functions are:

avg(...), sum(...), min(...), max(...)

count(*)

count(...), count(distinct ...), count(all...)

You can use arithmetic operators, concatenation, and recognized SQL functions in the select clause:

select cat.weight + sum(kitten.weight)from Cat cat join cat.kittens kittengroup by cat.id, cat.weight

select firstName||' '||initial||' '||upper(lastName) from Person

The distinct and all keywords can be used and have the same semantics as in SQL.

select distinct cat.name from Cat cat

select count(distinct cat.name), count(cat) from Cat cat

16.8. Polymorphic queries

A query like:

from Cat as cat

returns instances not only of Cat, but also of subclasses like DomesticCat. Hibernate queries can name any Java class or

interface in the from clause. The query will return instances of all persistent classes that extend that class or implement the

interface. The following query would return all persistent objects:

from java.lang.Object o

The interface Named might be implemented by various persistent classes:

from Named n, Named m where n.name = m.name

These last two queries will require more than one SQL SELECT. This means that the order by clause does not correctly

order the whole result set. It also means you cannot call these queries using Query.scroll().

16.9. The where clause

The where clause allows you to refine the list of instances returned. If no alias exists, you can refer to properties by name:

from Cat where name='Fritz'

If there is an alias, use a qualified property name:

from Cat as cat where cat.name='Fritz'

This returns instances of Cat named 'Fritz'.

The following query:

select foofrom Foo foo, Bar barwhere foo.startDate = bar.date

returns all instances of Foo with an instance of bar with a date property equal to the startDate property of the Foo. Compound

path expressions make the where clause extremely powerful. Consider the following:

from Cat cat where cat.mate.name is not null

This query translates to an SQL query with a table (inner) join. For example:

from Foo foowhere foo.bar.baz.customer.address.city is not null

would result in a query that would require four table joins in SQL.

The = operator can be used to compare not only properties, but also instances:

from Cat cat, Cat rival where cat.mate = rival.mate

select cat, matefrom Cat cat, Cat matewhere cat.mate = mate

The special property (lowercase) id can be used to reference the unique identifier of an object. See Section 16.5,

“Referring to identifier property” for more information.

from Cat as cat where cat.id = 123

from Cat as cat where cat.mate.id = 69

The second query is efficient and does not require a table join.

Properties of composite identifiers can also be used. Consider the following example where Person has composite

identifiers consisting of country and medicareNumber:

from bank.Person personwhere person.id.country = 'AU' and person.id.medicareNumber = 123456

from bank.Account accountwhere account.owner.id.country = 'AU' and account.owner.id.medicareNumber = 123456

Once again, the second query does not require a table join.

See Section 16.5, “Referring to identifier property” for more information regarding referencing identifier properties)

The special property class accesses the discriminator value of an instance in the case of polymorphic persistence. A Java

class name embedded in the where clause will be translated to its discriminator value.

from Cat cat where cat.class = DomesticCat

You can also use components or composite user types, or properties of said component types. See Section 16.17,“Components” for more information.

An "any" type has the special properties id and class that allows you to express a join in the following way (where

AuditLog.item is a property mapped with <any>):

from AuditLog log, Payment paymentwhere log.item.class = 'Payment' and log.item.id = payment.id

The log.item.class and payment.class would refer to the values of completely different database columns in the above query.

16.10. Expressions

Expressions used in the where clause include the following:

mathematical operators: +, -, *, /

binary comparison operators: =, >=, <=, <>, !=, like

logical operations and, or, not

Parentheses ( ) that indicates grouping

in, not in, between, is null, is not null, is empty, is not empty, member of and not member of

"Simple" case, case ... when ... then ... else ... end, and "searched" case, case when ... then ... else ... end

string concatenation ...||... or concat(...,...)

current_date(), current_time(), and current_timestamp()

second(...), minute(...), hour(...), day(...), month(...), and year(...)

Any function or operator defined by EJB-QL 3.0:substring(), trim(), lower(), upper(), length(), locate(), abs(), sqrt(), bit_length(), mod()

coalesce() and nullif()

str() for converting numeric or temporal values to a readable string

cast(... as ...), where the second argument is the name of a Hibernate type, and extract(... from ...) if ANSI cast() and

extract() is supported by the underlying database

the HQL index() function, that applies to aliases of a joined indexed collection

HQL functions that take collection-valued path expressions: size(), minelement(), maxelement(), minindex(), maxindex(),

along with the special elements() and indices functions that can be quantified using some, all, exists, any, in.

Any database-supported SQL scalar function like sign(), trunc(), rtrim(), and sin()

JDBC-style positional parameters ?

named parameters :name, :start_date, and :x1

SQL literals 'foo', 69, 6.66E+2, '1970-01-01 10:00:01.0'

Java public static final constants eg.Color.TABBY

in and between can be used as follows:

from DomesticCat cat where cat.name between 'A' and 'B'

from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )

The negated forms can be written as follows:

from DomesticCat cat where cat.name not between 'A' and 'B'

from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )

Similarly, is null and is not null can be used to test for null values.

Booleans can be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:

<property name="hibernate.query.substitutions">true 1, false 0</property>

This will replace the keywords true and false with the literals 1 and 0 in the translated SQL from this HQL:

from Cat cat where cat.alive = true

You can test the size of a collection with the special property size or the special size() function.

from Cat cat where cat.kittens.size > 0

from Cat cat where size(cat.kittens) > 0

For indexed collections, you can refer to the minimum and maximum indices using minindex and maxindex functions.

Similarly, you can refer to the minimum and maximum elements of a collection of basic type using the minelement and

maxelement functions. For example:

from Calendar cal where maxelement(cal.holidays) > current_date

from Order order where maxindex(order.items) > 100

from Order order where minelement(order.items) > 10000

The SQL functions any, some, all, exists, in are supported when passed the element or index set of a collection (elements and

indices functions) or the result of a subquery (see below):

select mother from Cat as mother, Cat as kitwhere kit in elements(foo.kittens)

select p from NameList list, Person pwhere p.name = some elements(list.names)

from Cat cat where exists elements(cat.kittens)

from Player p where 3 > all elements(p.scores)

from Show show where 'fizard' in indices(show.acts)

Note that these constructs - size, elements, indices, minindex, maxindex, minelement, maxelement - can only be used in the

where clause in Hibernate.

Elements of indexed collections (arrays, lists, and maps) can be referred to by index in a where clause only:

from Order order where order.items[0].id = 1234

select person from Person person, Calendar calendarwhere calendar.holidays['national day'] = person.birthDay and person.nationality.calendar = calendar

select item from Item item, Order orderwhere order.items[ order.deliveredItemIndices[0] ] = item and order.id = 11

select item from Item item, Order orderwhere order.items[ maxindex(order.items) ] = item and order.id = 11

The expression inside [] can even be an arithmetic expression:

select item from Item item, Order orderwhere order.items[ size(order.items) - 1 ] = item

HQL also provides the built-in index() function for elements of a one-to-many association or collection of values.

select item, index(item) from Order order join order.items itemwhere index(item) < 5

Scalar SQL functions supported by the underlying database can be used:

from DomesticCat cat where upper(cat.name) like 'FRI%'

Consider how much longer and less readable the following query would be in SQL:

select custfrom Product prod, Store store inner join store.customers custwhere prod.name = 'widget' and store.location.name in ( 'Melbourne', 'Sydney' ) and prod = all elements(cust.currentOrder.lineItems)

Hint: something like

SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_orderFROM customers cust, stores store, locations loc, store_customers sc, product prodWHERE prod.name = 'widget' AND store.loc_id = loc.id AND loc.name IN ( 'Melbourne', 'Sydney' ) AND sc.store_id = store.id AND sc.cust_id = cust.id AND prod.id = ALL( SELECT item.prod_id FROM line_items item, orders o WHERE item.order_id = o.id AND cust.current_order = o.id )

16.11. The order by clause

The list returned by a query can be ordered by any property of a returned class or components:

from DomesticCat catorder by cat.name asc, cat.weight desc nulls first, cat.birthdate

The optional asc or desc indicate ascending or descending order respectively.

The optional nulls first or nulls last indicate precedence of null values while sorting.

16.12. The group by clause

A query that returns aggregate values can be grouped by any property of a returned class or components:

select cat.color, sum(cat.weight), count(cat)from Cat catgroup by cat.color

select foo.id, avg(name), max(name)from Foo foo join foo.names namegroup by foo.id

A having clause is also allowed.

select cat.color, sum(cat.weight), count(cat)from Cat catgroup by cat.colorhaving cat.color in (eg.Color.TABBY, eg.Color.BLACK)

SQL functions and aggregate functions are allowed in the having and order by clauses if they are supported by the

underlying database (i.e., not in MySQL).

select catfrom Cat cat join cat.kittens kittengroup by cat.id, cat.name, cat.other, cat.propertieshaving avg(kitten.weight) > 100order by count(kitten) asc, sum(kitten.weight) desc

Neither the group by clause nor the order by clause can contain arithmetic expressions. Hibernate also does not currently

expand a grouped entity, so you cannot write group by cat if all properties o f cat are non-aggregated. You have to list all

non-aggregated properties explicitly.

16.13. Subqueries

For databases that support subselects, Hibernate supports subqueries within queries. A subquery must be surrounded byparentheses (often by an SQL aggregate function call). Even correlated subqueries (subqueries that refer to an alias in theouter query) are allowed.

from Cat as fatcatwhere fatcat.weight > ( select avg(cat.weight) from DomesticCat cat)

from DomesticCat as catwhere cat.name = some ( select name.nickName from Name as name)

from Cat as catwhere not exists ( from Cat as mate where mate.mate = cat)

from DomesticCat as catwhere cat.name not in ( select name.nickName from Name as name)

select cat.id, (select max(kit.weight) from cat.kitten kit)from Cat as cat

Note that HQL subqueries can occur only in the select or where clauses.

Note that subqueries can also utilize row value constructor syntax. See Section 16.18, “Row value constructor syntax” for

more information.

16.14. HQL examples

Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hibernate's mainstrengths. The following example queries are similar to queries that have been used on recent projects. Please note thatmost queries you will write will be much simpler than the following examples.

The following query returns the order id, number of items, the given minimum total value and the total value of the order forall unpaid orders for a particular customer. The results are ordered by total value. In determining the prices, it uses thecurrent catalog. The resulting SQL query, against the ORDER, ORDER_LINE, PRODUCT, CATALOG and PRICE tables has

four inner joins and an (uncorrelated) subselect.

select order.id, sum(price.amount), count(item)from Order as order join order.lineItems as item join item.product as product, Catalog as catalog join catalog.prices as pricewhere order.paid = false and order.customer = :customer and price.product = product and catalog.effectiveDate < sysdate and catalog.effectiveDate >= all ( select cat.effectiveDate from Catalog as cat where cat.effectiveDate < sysdate )group by orderhaving sum(price.amount) > :minAmountorder by sum(price.amount) desc

What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like this:

select order.id, sum(price.amount), count(item)from Order as order join order.lineItems as item join item.product as product, Catalog as catalog join catalog.prices as pricewhere order.paid = false and order.customer = :customer and price.product = product and catalog = :currentCataloggroup by orderhaving sum(price.amount) > :minAmountorder by sum(price.amount) desc

The next query counts the number of payments in each status, excluding all payments in the AWAITING_APPROVAL status

where the most recent status change was made by the current user. It translates to an SQL query with two inner joins anda correlated subselect against the PAYMENT, PAYMENT_STATUS and PAYMENT_STATUS_CHANGE tables.

select count(payment), status.namefrom Payment as payment join payment.currentStatus as status join payment.statusChanges as statusChangewhere payment.status.name <> PaymentStatus.AWAITING_APPROVAL or ( statusChange.timeStamp = ( select max(change.timeStamp) from PaymentStatusChange change where change.payment = payment ) and statusChange.user <> :currentUser )group by status.name, status.sortOrderorder by status.sortOrder

If the statusChanges collection was mapped as a list, instead of a set, the query would have been much simpler to write.

select count(payment), status.namefrom Payment as payment join payment.currentStatus as statuswhere payment.status.name <> PaymentStatus.AWAITING_APPROVAL or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <> :currentUsergroup by status.name, status.sortOrderorder by status.sortOrder

The next query uses the MS SQL Server isNull() function to return all the accounts and unpaid payments for the

organization to which the current user belongs. It translates to an SQL query with three inner joins, an outer join and asubselect against the ACCOUNT, PAYMENT, PAYMENT_STATUS, ACCOUNT_TYPE, ORGANIZATION and ORG_USER tables.

select account, paymentfrom Account as account left outer join account.payments as paymentwhere :currentUser in elements(account.holder.users) and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)order by account.type.sortOrder, account.accountNumber, payment.dueDate

For some databases, we would need to do away with the (correlated) subselect.

select account, paymentfrom Account as account join account.holder.users as user left outer join account.payments as paymentwhere :currentUser = user and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)order by account.type.sortOrder, account.accountNumber, payment.dueDate

16.15. Bulk update and delete

HQL now supports update, delete and insert ... select ... statements. See Section 15.4, “DML-style operations” for more

information.

16.16. Tips & Tricks

You can count the number of query results without returning them:

( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()

To order a result by the size of a collection, use the following query:

select usr.id, usr.namefrom User as usr left join usr.messages as msggroup by usr.id, usr.nameorder by count(msg)

If your database supports subselects, you can place a condition upon selection size in the where clause of your query:

from User usr where size(usr.messages) >= 1

If your database does not support subselects, use the following query:

select usr.id, usr.namefrom User usr join usr.messages msggroup by usr.id, usr.namehaving count(msg) >= 1

As this solution cannot return a User with zero messages because of the inner join, the following form is also useful:

select usr.id, usr.namefrom User as usr left join usr.messages as msggroup by usr.id, usr.namehaving count(msg) = 0

Properties of a JavaBean can be bound to named query parameters:

Query q = s.createQuery("from foo Foo as foo where foo.name=:name and foo.size=:size");q.setProperties(fooBean); // fooBean has getName() and getSize()List foos = q.list();

Collections are pageable by using the Query interface with a filter:

Query q = s.createFilter( collection, "" ); // the trivial filterq.setMaxResults(PAGE_SIZE);q.setFirstResult(PAGE_SIZE * pageNumber);List page = q.list();

Collection elements can be ordered or grouped using a query filter:

Collection orderedCollection = s.filter( collection, "order by this.amount" );Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );

You can find the size of a collection without initializing it:

( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();

16.17. Components

Components can be used similarly to the simple value types that are used in HQL queries. They can appear in the select

clause as follows:

select p.name from Person p

select p.name.first from Person p

where the Person's name property is a component. Components can also be used in the where clause:

from Person p where p.name = :name

from Person p where p.name.first = :firstName

Components can also be used in the order by clause:

from Person p order by p.name

from Person p order by p.name.first

Another common use of components is in Section 16.18, “Row value constructor syntax”.

16.18. Row value constructor syntax

HQL supports the use of ANSI SQL row value constructor syntax, sometimes referred to AS tuple syntax, even though the

underlying database may not support that notion. Here, we are generally referring to multi-valued comparisons, typicallyassociated with components. Consider an entity Person which defines a name component:

from Person p where p.name.first='John' and p.name.last='Jingleheimer-Schmidt'

That is valid syntax although it is a little verbose. You can make this more concise by using row value constructor syntax:

from Person p where p.name=('John', 'Jingleheimer-Schmidt')

It can also be useful to specify this in the select clause:

select p.name from Person p

Using row value constructor syntax can also be beneficial when using subqueries that need to compare against multiple

values:

from Cat as catwhere not ( cat.name, cat.color ) in ( select cat.name, cat.color from DomesticCat cat)

One thing to consider when deciding if you want to use this syntax, is that the query will be dependent upon the orderingof the component sub-properties in the metadata.

Chapter 17. Criteria Queries

Table of Contents


17.2. Narrowing the result set17.3. Ordering the results17.4. Associations17.5. Dynamic association fetching17.6. Components17.7. Collections17.8. Example queries17.9. Projections, aggregation and grouping17.10. Detached queries and subqueries17.11. Queries by natural identifier

Hibernate features an intuitive, extensible criteria query API.


The interface org.hibernate.Criteria represents a query against a particular persistent class. The Session is a factory for Criteria

instances.

Criteria crit = sess.createCriteria(Cat.class);crit.setMaxResults(50);List cats = crit.list();

17.2. Narrowing the result set

An individual query criterion is an instance of the interface org.hibernate.criterion.Criterion. The class

org.hibernate.criterion.Restrictions defines factory methods for obtaining certain built-in Criterion types.

List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .add( Restrictions.between("weight", minWeight, maxWeight) ) .list();

Restrictions can be grouped logically.

List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .add( Restrictions.or( Restrictions.eq( "age", new Integer(0) ), Restrictions.isNull("age") ) ) .list();

List cats = sess.createCriteria(Cat.class) .add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) ) .add( Restrictions.disjunction() .add( Restrictions.isNull("age") ) .add( Restrictions.eq("age", new Integer(0) ) ) .add( Restrictions.eq("age", new Integer(1) ) ) .add( Restrictions.eq("age", new Integer(2) ) ) ) ) .list();

There are a range of built-in criterion types (Restrictions subclasses). One of the most useful allows you to specify SQL

directly.

List cats = sess.createCriteria(Cat.class) .add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) ) .list();

The {alias} placeholder will be replaced by the row alias of the queried entity.

You can also obtain a criterion from a Property instance. You can create a Property by calling Property.forName():

Property age = Property.forName("age");List cats = sess.createCriteria(Cat.class) .add( Restrictions.disjunction() .add( age.isNull() ) .add( age.eq( new Integer(0) ) ) .add( age.eq( new Integer(1) ) ) .add( age.eq( new Integer(2) ) ) ) ) .add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) ) .list();

17.3. Ordering the results

You can order the results using org.hibernate.criterion.Order.

List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "F%") .addOrder( Order.asc("name") ) .addOrder( Order.desc("age") ) .setMaxResults(50) .list();

List cats = sess.createCriteria(Cat.class) .add( Property.forName("name").like("F%") ) .addOrder( Property.forName("name").asc() ) .addOrder( Property.forName("age").desc() ) .setMaxResults(50) .list();

17.4. Associations

By navigating associations using createCriteria() you can specify constraints upon related entities:

List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "F%") ) .createCriteria("kittens") .add( Restrictions.like("name", "F%") ) .list();

The second createCriteria() returns a new instance of Criteria that refers to the elements of the kittens collection.

There is also an alternate form that is useful in certain circumstances:

List cats = sess.createCriteria(Cat.class) .createAlias("kittens", "kt") .createAlias("mate", "mt") .add( Restrictions.eqProperty("kt.name", "mt.name") ) .list();

(createAlias() does not create a new instance of Criteria.)

The kittens collections held by the Cat instances returned by the previous two queries are not pre-filtered by the criteria. If

you want to retrieve just the kittens that match the criteria, you must use a ResultTransformer.

List cats = sess.createCriteria(Cat.class) .createCriteria("kittens", "kt") .add( Restrictions.eq("name", "F%") ) .setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP) .list();Iterator iter = cats.iterator();while ( iter.hasNext() ) { Map map = (Map) iter.next(); Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS); Cat kitten = (Cat) map.get("kt");}

Additionally you may manipulate the result set using a left outer join:

List cats = session.createCriteria( Cat.class ) .createAlias("mate", "mt", Criteria.LEFT_JOIN, Restrictions.like("mt.name", "good%") ) .addOrder(Order.asc("mt.age")) .list();

This will return all of the Cats with a mate whose name starts with "good" ordered by their mate's age, and all cats who do

not have a mate. This is useful when there is a need to order or limit in the database prior to returning complex/large resultsets, and removes many instances where multiple queries would have to be performed and the results unioned by java inmemory.

Without this feature, first all of the cats without a mate would need to be loaded in one query.

A second query would need to retreive the cats with mates who's name started with "good" sorted by the mates age.

Thirdly, in memory; the lists would need to be joined manually.

17.5. Dynamic association fetching

You can specify association fetching semantics at runtime using setFetchMode().

List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .setFetchMode("mate", FetchMode.EAGER) .setFetchMode("kittens", FetchMode.EAGER) .list();

This query will fetch both mate and kittens by outer join. See Section 20.1, “Fetching strategies” for more information.

17.6. Components

To add a restriction against a property of an embedded component, the component property name should be prependedto the property name when creating the Restriction. The criteria object should be created on the owning entity, and cannot

be created on the component itself. For example, suppose the Cat has a component property fullName with sub-properties

firstName and lastName:

List cats = session.createCriteria(Cat.class) .add(Restrictions.eq("fullName.lastName", "Cattington")) .list();

Note: this does not apply when querying collections of components, for that see below Section 17.7, “Collections”

17.7. Collections

When using criteria against collections, there are two distinct cases. One is if the collection contains entities (eg.<one-to-many/> or <many-to-many/>) or components (<composite-element/> ), and the second is if the collection contains scalar

values (<element/>). In the first case, the syntax is as given above in the section Section 17.4, “Associations” where we

restrict the kittens collection. Essentially we create a Criteria object against the collection property and restrict the entity or

component properties using that instance.

For queryng a collection of basic values, we still create the Criteria object against the collection, but to reference the value,

we use the special property "elements". For an indexed collection, we can also reference the index property using thespecial property "indices".

List cats = session.createCriteria(Cat.class) .createCriteria("nickNames") .add(Restrictions.eq("elements", "BadBoy")) .list();

17.8. Example queries

The class org.hibernate.criterion.Example allows you to construct a query criterion from a given instance.

Cat cat = new Cat();cat.setSex('F');cat.setColor(Color.BLACK);List results = session.createCriteria(Cat.class) .add( Example.create(cat) ) .list();

Version properties, identifiers and associations are ignored. By default, null valued properties are excluded.

You can adjust how the Example is applied.

Example example = Example.create(cat) .excludeZeroes() //exclude zero valued properties .excludeProperty("color") //exclude the property named "color" .ignoreCase() //perform case insensitive string comparisons .enableLike(); //use like for string comparisonsList results = session.createCriteria(Cat.class) .add(example) .list();

You can even use examples to place criteria upon associated objects.

List results = session.createCriteria(Cat.class) .add( Example.create(cat) ) .createCriteria("mate") .add( Example.create( cat.getMate() ) ) .list();

17.9. Projections, aggregation and grouping

The class org.hibernate.criterion.Projections is a factory for Projection instances. You can apply a projection to a query by

calling setProjection().

List results = session.createCriteria(Cat.class) .setProjection( Projections.rowCount() ) .add( Restrictions.eq("color", Color.BLACK) ) .list();

List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount() ) .add( Projections.avg("weight") ) .add( Projections.max("weight") ) .add( Projections.groupProperty("color") ) ) .list();

There is no explicit "group by" necessary in a criteria query. Certain projection types are defined to be groupingprojections, which also appear in the SQL group by clause.

An alias can be assigned to a projection so that the projected value can be referred to in restrictions or orderings. Here aretwo different ways to do this:

List results = session.createCriteria(Cat.class) .setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) ) .addOrder( Order.asc("colr") ) .list();

List results = session.createCriteria(Cat.class) .setProjection( Projections.groupProperty("color").as("colr") ) .addOrder( Order.asc("colr") ) .list();

The alias() and as() methods simply wrap a projection instance in another, aliased, instance of Projection. As a shortcut, you

can assign an alias when you add the projection to a projection list:

List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount(), "catCountByColor" ) .add( Projections.avg("weight"), "avgWeight" ) .add( Projections.max("weight"), "maxWeight" ) .add( Projections.groupProperty("color"), "color" ) ) .addOrder( Order.desc("catCountByColor") ) .addOrder( Order.desc("avgWeight") ) .list();

List results = session.createCriteria(Domestic.class, "cat") .createAlias("kittens", "kit") .setProjection( Projections.projectionList() .add( Projections.property("cat.name"), "catName" ) .add( Projections.property("kit.name"), "kitName" ) ) .addOrder( Order.asc("catName") ) .addOrder( Order.asc("kitName") ) .list();

You can also use Property.forName() to express projections:

List results = session.createCriteria(Cat.class) .setProjection( Property.forName("name") ) .add( Property.forName("color").eq(Color.BLACK) ) .list();

List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount().as("catCountByColor") ) .add( Property.forName("weight").avg().as("avgWeight") ) .add( Property.forName("weight").max().as("maxWeight") ) .add( Property.forName("color").group().as("color" ) ) .addOrder( Order.desc("catCountByColor") ) .addOrder( Order.desc("avgWeight") ) .list();

17.10. Detached queries and subqueries

The DetachedCriteria class allows you to create a query outside the scope of a session and then execute it using an

arbitrary Session.

DetachedCriteria query = DetachedCriteria.forClass(Cat.class) .add( Property.forName("sex").eq('F') ); Session session = ....;Transaction txn = session.beginTransaction();List results = query.getExecutableCriteria(session).setMaxResults(100).list();txn.commit();session.close();

A DetachedCriteria can also be used to express a subquery. Criterion instances involving subqueries can be obtained via

Subqueries or Property.

DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class) .setProjection( Property.forName("weight").avg() );session.createCriteria(Cat.class) .add( Property.forName("weight").gt(avgWeight) ) .list();

DetachedCriteria weights = DetachedCriteria.forClass(Cat.class) .setProjection( Property.forName("weight") );session.createCriteria(Cat.class) .add( Subqueries.geAll("weight", weights) ) .list();

Correlated subqueries are also possible:

DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2") .setProjection( Property.forName("weight").avg() ) .add( Property.forName("cat2.sex").eqProperty("cat.sex") );session.createCriteria(Cat.class, "cat") .add( Property.forName("weight").gt(avgWeightForSex) ) .list();

Example of multi-column restriction based on a subquery:

DetachedCriteria sizeQuery = DetachedCriteria.forClass( Man.class ) .setProjection( Projections.projectionList().add( Projections.property( "weight" ) ) .add( Projections.property( "height" ) ) ) .add( Restrictions.eq( "name", "John" ) );session.createCriteria( Woman.class ) .add( Subqueries.propertiesEq( new String[] { "weight", "height" }, sizeQuery ) ) .list();

17.11. Queries by natural identifier

For most queries, including criteria queries, the query cache is not efficient because query cache invalidation occurs toofrequently. However, there is a special kind of query where you can optimize the cache invalidation algorithm: lookups bya constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides specialprovision for this use case.

First, map the natural key of your entity using <natural-id> and enable use of the second-level cache.

<class name="User"> <cache usage="read-write"/> <id name="id"> <generator class="increment"/> </id> <natural-id> <property name="name"/> <property name="org"/> </natural-id> <property name="password"/></class>

This functionality is not intended for use with entities with mutable natural keys.

Once you have enabled the Hibernate query cache, the Restrictions.naturalId() allows you to make use of the more efficient

cache algorithm.

session.createCriteria(User.class) .add( Restrictions.naturalId() .set("name", "gavin") .set("org", "hb") ).setCacheable(true) .uniqueResult();

Chapter 18. Native SQL

Table of Contents


18.1.1. Scalar queries18.1.2. Entity queries18.1.3. Handling associations and collections18.1.4. Returning multiple entities18.1.5. Returning non-managed entities18.1.6. Handling inheritance18.1.7. Parameters


18.2.1. Using return-property to explicitly specify column/alias names18.2.2. Using stored procedures for querying

18.3. Custom SQL for create, update and delete18.4. Custom SQL for loading

You can also express queries in the native SQL dialect of your database. This is useful if you want to utilize database-specific features such as query hints or the CONNECT keyword in Oracle. It also provides a clean migration path from a

direct SQL/JDBC based application to Hibernate.

Hibernate allows you to specify handwritten SQL, including stored procedures, for all create, update, delete, and loadoperations.


Execution of native SQL queries is controlled via the SQLQuery interface, which is obtained by calling

Session.createSQLQuery(). The following sections describe how to use this API for querying.

18.1.1. Scalar queries

The most basic SQL query is to get a list of scalars (values).

sess.createSQLQuery("SELECT * FROM CATS").list();sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();

These will return a List of Object arrays (Object[]) with scalar values for each column in the CATS table. Hibernate will useResultSetMetadata to deduce the actual order and types of the returned scalar values.

To avoid the overhead of using ResultSetMetadata, or simply to be more explicit in what is returned, one can use addScalar():

sess.createSQLQuery("SELECT * FROM CATS") .addScalar("ID", Hibernate.LONG) .addScalar("NAME", Hibernate.STRING) .addScalar("BIRTHDATE", Hibernate.DATE)

This query specified:

the SQL query string

the columns and types to return

This will return Object arrays, but now it will not use ResultSetMetadata but will instead explicitly get the ID, NAME and

BIRTHDATE column as respectively a Long, String and a Short from the underlying resultset. This also means that onlythese three columns will be returned, even though the query is using * and could return more than the three listed

columns.

It is possible to leave out the type information for all or some of the scalars.

sess.createSQLQuery("SELECT * FROM CATS") .addScalar("ID", Hibernate.LONG) .addScalar("NAME") .addScalar("BIRTHDATE")

This is essentially the same query as before, but now ResultSetMetaData is used to determine the type of NAME and

BIRTHDATE, where as the type of ID is explicitly specified.

How the java.sql.Types returned from ResultSetMetaData is mapped to Hibernate types is controlled by the Dialect. If aspecific type is not mapped, or does not result in the expected type, it is possible to customize it via calls toregisterHibernateType in the Dialect.

18.1.2. Entity queries

The above queries were all about returning scalar values, basically returning the "raw" values from the resultset. Thefollowing shows how to get entity objects from a native sql query via addEntity().

sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);



the entity returned by the query

Assuming that Cat is mapped as a class with the columns ID, NAME and BIRTHDATE the above queries will both return aList where each element is a Cat entity.

If the entity is mapped with a many-to-one to another entity it is required to also return this when performing the native

query, otherwise a database specific "column not found" error will occur. The additional columns will automatically bereturned when using the * notation, but we prefer to be explicit as in the following example for a many-to-one to a Dog:

sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM CATS").addEntity(Cat.class);

This will allow cat.getDog() to function properly.

18.1.3. Handling associations and collections

It is possible to eagerly join in the Dog to avoid the possible extra roundtrip for initializing the proxy. This is done via the

addJoin() method, which allows you to join in an association or collection.

sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID, D_NAME FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID") .addEntity("cat", Cat.class) .addJoin("cat.dog");

In this example, the returned Cat's will have their dog property fully initialized without any extra roundtrip to the database.

Notice that you added an alias name ("cat") to be able to specify the target property path of the join. It is possible to do thesame eager joining for collections, e.g. if the Cat had a one-to-many to Dog instead.

sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME, CAT_ID FROM CATS c, DOGS d WHERE c.ID = d.CAT_ID") .addEntity("cat", Cat.class) .addJoin("cat.dogs");

At this stage you are reaching the limits of what is possible with native queries, without starting to enhance the sql queriesto make them usable in Hibernate. Problems can arise when returning multiple entities of the same type or when thedefault alias/column names are not enough.

18.1.4. Returning multiple entities

Until now, the result set column names are assumed to be the same as the column names specified in the mappingdocument. This can be problematic for SQL queries that join multiple tables, since the same column names can appear inmore than one table.

Column alias injection is needed in the following query (which most likely will fail):

sess.createSQLQuery("SELECT c.*, m.* FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID") .addEntity("cat", Cat.class) .addEntity("mother", Cat.class)

The query was intended to return two Cat instances per row: a cat and its mother. The query will, however, fail becausethere is a conflict of names; the instances are mapped to the same column names. Also, on some databases the returnedcolumn aliases will most likely be on the form "c.ID", "c.NAME", etc. which are not equal to the columns specified in themappings ("ID" and "NAME").

The following form is not vulnerable to column name duplication:

sess.createSQLQuery("SELECT {cat.*}, {m.*} FROM CATS c, CATS m WHERE c.MOTHER_ID = m.ID") .addEntity("cat", Cat.class) .addEntity("mother", Cat.class)


the SQL query string, with placeholders for Hibernate to inject column aliases

the entities returned by the query

The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you can list the columnsexplicitly, but even in this case Hibernate injects the SQL column aliases for each property. The placeholder for a columnalias is just the property name qualified by the table alias. In the following example, you retrieve Cats and their mothersfrom a different table (cat_log) to the one declared in the mapping metadata. You can even use the property aliases in thewhere clause.

String sql = "SELECT ID as {c.id}, NAME as {c.name}, " + "BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*} " + "FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";

List loggedCats = sess.createSQLQuery(sql) .addEntity("cat", Cat.class) .addEntity("mother", Cat.class).list()

18.1.4.1. Alias and property references

In most cases the above alias injection is needed. For queries relating to more complex mappings, like compositeproperties, inheritance discriminators, collections etc., you can use specific aliases that allow Hibernate to inject theproper aliases.

The following table shows the different ways you can use the alias injection. Please note that the alias names in the resultare simply examples; each alias will have a unique and probably different name when used.

Table 18.1. Alias injection names

Description Syntax Example

A simpleproperty

{[aliasname].[propertyname]} A_NAME as {item.name}

A compositeproperty

{[aliasname].[componentname].[propertyname]} CURRENCY as {item.amount.currency}, VALUE as {item.amount.value}

Discriminatorof an entity

{[aliasname].class} DISC as {item.class}

All propertiesof an entity

{[aliasname].*} {item.*}

A collectionkey

{[aliasname].key} ORGID as {coll.key}

The id of ancollection

{[aliasname].id} EMPID as {coll.id}

The elementof ancollection

{[aliasname].element} XID as {coll.element}

property ofthe elementin thecollection

{[aliasname].element.[propertyname]} NAME as {coll.element.name}

All propertiesof theelement inthe collection

{[aliasname].element.*} {coll.element.*}

All propertiesof thecollection

{[aliasname].*} {coll.*}

18.1.5. Returning non-managed entities

It is possible to apply a ResultTransformer to native SQL queries, allowing it to return non-managed entities.

sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS") .setResultTransformer(Transformers.aliasToBean(CatDTO.class))



a result transformer

The above query will return a list of CatDTO which has been instantiated and injected the values of NAME and

BIRTHNAME into its corresponding properties or fields.

18.1.6. Handling inheritance

Native SQL queries which query for entities that are mapped as part of an inheritance must include all properties for thebaseclass and all its subclasses.

18.1.7. Parameters

Native SQL queries support positional as well as named parameters:

Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like ?").addEntity(Cat.class);List pusList = query.setString(0, "Pus%").list(); query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like :name").addEntity(Cat.class);List pusList = query.setString("name", "Pus%").list();


Named SQL queries can also be defined in the mapping document and called in exactly the same way as a named HQLquery (see Section 11.4.1.7, “Externalizing named queries”). In this case, you do not need to call addEntity().

Example 18.1. Named sql query using the <sql-query> maping element

<sql-query name="persons"> <return alias="person" class="eg.Person"/> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex} FROM PERSON person WHERE person.NAME LIKE :namePattern</sql-query>

Example 18.2. Execution of a named query

List people = sess.getNamedQuery("persons") .setString("namePattern", namePattern) .setMaxResults(50) .list();

The <return-join> element is use to join associations and the <load-collection> element is used to define queries which

initialize collections,

Example 18.3. Named sql query with association

<sql-query name="personsWith"> <return alias="person" class="eg.Person"/> <return-join alias="address" property="person.mailingAddress"/> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex}, address.STREET AS {address.street}, address.CITY AS {address.city}, address.STATE AS {address.state}, address.ZIP AS {address.zip} FROM PERSON person JOIN ADDRESS address ON person.ID = address.PERSON_ID AND address.TYPE='MAILING' WHERE person.NAME LIKE :namePattern</sql-query>

A named SQL query may return a scalar value. You must declare the column alias and Hibernate type using the<return-scalar> element:

Example 18.4. Named query returning a scalar

<sql-query name="mySqlQuery"> <return-scalar column="name" type="string"/> <return-scalar column="age" type="long"/> SELECT p.NAME AS name, p.AGE AS age, FROM PERSON p WHERE p.NAME LIKE 'Hiber%'</sql-query>

You can externalize the resultset mapping information in a <resultset> element which will allow you to either reuse them

across several named queries or through the setResultSetMapping() API.

Example 18.5. <resultset> mapping used to externalize mapping information

<resultset name="personAddress"> <return alias="person" class="eg.Person"/> <return-join alias="address" property="person.mailingAddress"/></resultset>

<sql-query name="personsWith" resultset-ref="personAddress"> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex}, address.STREET AS {address.street}, address.CITY AS {address.city}, address.STATE AS {address.state}, address.ZIP AS {address.zip} FROM PERSON person JOIN ADDRESS address ON person.ID = address.PERSON_ID AND address.TYPE='MAILING' WHERE person.NAME LIKE :namePattern</sql-query>

You can, alternatively, use the resultset mapping information in your hbm files directly in java code.

Example 18.6. Programmatically specifying the result mapping information

List cats = sess.createSQLQuery( "select {cat.*}, {kitten.*} from cats cat, cats kitten where kitten.mother = cat.id" ) .setResultSetMapping("catAndKitten") .list();

So far we have only looked at externalizing SQL queries using Hibernate mapping files. The same concept is alsoavailable with anntations and is called named native queries. You can use @NamedNativeQuery (@NamedNativeQueries) in

conjunction with @SqlResultSetMapping (@SqlResultSetMappings). Like @NamedQuery, @NamedNativeQuery and

@SqlResultSetMapping can be defined at class level, but their scope is global to the application. Lets look at a view

examples.

Example 18.7, “Named SQL query using @NamedNativeQuery together with @SqlResultSetMapping” shows how a

resultSetMapping parameter is defined in @NamedNativeQuery. It represents the name of a defined @SqlResultSetMapping.

The resultset mapping declares the entities retrieved by this native query. Each field of the entity is bound to an SQL alias(or column name). All fields of the entity including the ones of subclasses and the foreign key columns of related entitieshave to be present in the SQL query. Field definitions are optional provided that they map to the same column name asthe one declared on the class property. In the example 2 entities, Night and Area, are returned and each property is

declared and associated to a column name, actually the column name retrieved by the query.

In Example 18.8, “Implicit result set mapping” the result set mapping is implicit. We only describe the entity class of theresult set mapping. The property / column mappings is done using the entity mapping values. In this case the modelproperty is bound to the model_txt column.

Finally, if the association to a related entity involve a composite primary key, a @FieldResult element should be used for

each foreign key column. The @FieldResult name is composed of the property name for the relationship, followed by a dot

("."), followed by the name or the field or property of the primary key. This can be seen in Example 18.9, “Using dotnotation in @FieldResult for specifying associations ”.

Example 18.7. Named SQL query using @NamedNativeQuery together with @SqlResultSetMapping

@NamedNativeQuery(name="night&area", query="select night.id nid, night.night_duration, " + " night.night_date, area.id aid, night.area_id, area.name " + "from Night night, Area area where night.area_id = area.id", resultSetMapping="joinMapping")@SqlResultSetMapping(name="joinMapping", entities={ @EntityResult(entityClass=Night.class, fields = { @FieldResult(name="id", column="nid"), @FieldResult(name="duration", column="night_duration"), @FieldResult(name="date", column="night_date"), @FieldResult(name="area", column="area_id"), discriminatorColumn="disc" }), @EntityResult(entityClass=org.hibernate.test.annotations.query.Area.class, fields = { @FieldResult(name="id", column="aid"), @FieldResult(name="name", column="name") }) })

Example 18.8. Implicit result set mapping

@Entity@SqlResultSetMapping(name="implicit", entities=@EntityResult(entityClass=SpaceShip.class))@NamedNativeQuery(name="implicitSample", query="select * from SpaceShip", resultSetMapping="implicit")public class SpaceShip { private String name; private String model; private double speed;

@Id public String getName() { return name; }

public void setName(String name) { this.name = name; }

@Column(name="model_txt") public String getModel() { return model; }

public void setModel(String model) { this.model = model; }

public double getSpeed() { return speed; }

public void setSpeed(double speed) { this.speed = speed; }}

Example 18.9. Using dot notation in @FieldResult for specifying associations

@Entity@SqlResultSetMapping(name="compositekey", entities=@EntityResult(entityClass=SpaceShip.class, fields = { @FieldResult(name="name", column = "name"), @FieldResult(name="model", column = "model"), @FieldResult(name="speed", column = "speed"), @FieldResult(name="captain.firstname", column = "firstn"), @FieldResult(name="captain.lastname", column = "lastn"), @FieldResult(name="dimensions.length", column = "length"), @FieldResult(name="dimensions.width", column = "width") }), columns = { @ColumnResult(name = "surface"), @ColumnResult(name = "volume") } )

@NamedNativeQuery(name="compositekey", query="select name, model, speed, lname as lastn, fname as firstn, length, width, length * width as surface from SpaceShip", resultSetMapping="compositekey")} )public class SpaceShip { private String name; private String model; private double speed; private Captain captain; private Dimensions dimensions;

@Id public String getName() { return name; }

public void setName(String name) { this.name = name; }

@ManyToOne(fetch= FetchType.LAZY) @JoinColumns( { @JoinColumn(name="fname", referencedColumnName = "firstname"), @JoinColumn(name="lname", referencedColumnName = "lastname") } ) public Captain getCaptain() { return captain; }

public void setCaptain(Captain captain) { this.captain = captain; }

public String getModel() { return model; }

public void setModel(String model) { this.model = model; }

public double getSpeed() { return speed; }

public void setSpeed(double speed) { this.speed = speed; }

public Dimensions getDimensions() { return dimensions; }

public void setDimensions(Dimensions dimensions) { this.dimensions = dimensions; }}

@Entity@IdClass(Identity.class)public class Captain implements Serializable { private String firstname; private String lastname;

@Id public String getFirstname() { return firstname; }

public void setFirstname(String firstname) { this.firstname = firstname; }

@Id public String getLastname() { return lastname; }

public void setLastname(String lastname) { this.lastname = lastname; }}

Tip

If you retrieve a single entity using the default mapping, you can specify the resultClass

attribute instead of resultSetMapping:

@NamedNativeQuery(name="implicitSample", query="select * from SpaceShip", resultClass=SpaceShip.class)public class SpaceShip {

In some of your native queries, you'll have to return scalar values, for example when building report queries. You can mapthem in the @SqlResultsetMapping through @ColumnResult. You actually can even mix, entities and scalar returns in the

same native query (this is probably not that common though).

Example 18.10. Scalar values via @ColumnResult

@SqlResultSetMapping(name="scalar", columns=@ColumnResult(name="dimension"))@NamedNativeQuery(name="scalar", query="select length*width as dimension from SpaceShip", resultSetMapping="scalar")

An other query hint specific to native queries has been introduced: org.hibernate.callable which can be true or false

depending on whether the query is a stored procedure or not.

18.2.1. Using return-property to explicitly specify column/alias names

You can explicitly tell Hibernate what column aliases to use with <return-property>, instead of using the {}-syntax to let

Hibernate inject its own aliases.For example:

<sql-query name="mySqlQuery"> <return alias="person" class="eg.Person"> <return-property name="name" column="myName"/> <return-property name="age" column="myAge"/> <return-property name="sex" column="mySex"/> </return> SELECT person.NAME AS myName, person.AGE AS myAge, person.SEX AS mySex, FROM PERSON person WHERE person.NAME LIKE :name</sql-query>

<return-property> also works with multiple columns. This solves a limitation with the {}-syntax which cannot allow fine

grained control of multi-column properties.

<sql-query name="organizationCurrentEmployments"> <return alias="emp" class="Employment"> <return-property name="salary"> <return-column name="VALUE"/> <return-column name="CURRENCY"/> </return-property> <return-property name="endDate" column="myEndDate"/> </return> SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS {emp.employer}, STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate}, REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE, CURRENCY FROM EMPLOYMENT WHERE EMPLOYER = :id AND ENDDATE IS NULL ORDER BY STARTDATE ASC</sql-query>

In this example <return-property> was used in combination with the {}-syntax for injection. This allows users to choose how

they want to refer column and properties.

If your mapping has a discriminator you must use <return-discriminator> to specify the discriminator column.

18.2.2. Using stored procedures for querying

Hibernate provides support for queries via stored procedures and functions. Most of the following documentation isequivalent for both. The stored procedure/function must return a resultset as the first out-parameter to be able to work withHibernate. An example of such a stored function in Oracle 9 and higher is as follows:

CREATE OR REPLACE FUNCTION selectAllEmployments RETURN SYS_REFCURSORAS st_cursor SYS_REFCURSOR;BEGIN OPEN st_cursor FOR SELECT EMPLOYEE, EMPLOYER, STARTDATE, ENDDATE, REGIONCODE, EID, VALUE, CURRENCY FROM EMPLOYMENT; RETURN st_cursor; END;

To use this query in Hibernate you need to map it via a named query.

<sql-query name="selectAllEmployees_SP" callable="true"> <return alias="emp" class="Employment"> <return-property name="employee" column="EMPLOYEE"/> <return-property name="employer" column="EMPLOYER"/> <return-property name="startDate" column="STARTDATE"/> <return-property name="endDate" column="ENDDATE"/> <return-property name="regionCode" column="REGIONCODE"/> <return-property name="id" column="EID"/> <return-property name="salary"> <return-column name="VALUE"/> <return-column name="CURRENCY"/> </return-property> </return> { ? = call selectAllEmployments() }</sql-query>

Stored procedures currently only return scalars and entities. <return-join> and <load-collection> are not supported.

18.2.2.1. Rules/limitations for using stored procedures

You cannot use stored procedures with Hibernate unless you follow some procedure/function rules. If they do not followthose rules they are not usable with Hibernate. If you still want to use these procedures you have to execute them viasession.connection(). The rules are different for each database, since database vendors have different stored procedure

semantics/syntax.

Stored procedure queries cannot be paged with setFirstResult()/setMaxResults().

The recommended call form is standard SQL92: { ? = call functionName(<parameters>) } or

{ ? = call procedureName(<parameters>}. Native call syntax is not supported.

For Oracle the following rules apply:

A function must return a result set. The first parameter of a procedure must be an OUT that returns a result set. This is

done by using a SYS_REFCURSOR type in Oracle 9 or 10. In Oracle you need to define a REF CURSOR type. See

Oracle literature for further information.

For Sybase or MS SQL server the following rules apply:

The procedure must return a result set. Note that since these servers can return multiple result sets and update counts,Hibernate will iterate the results and take the first result that is a result set as its return value. Everything else will bediscarded.

If you can enable SET NOCOUNT ON in your procedure it will probably be more efficient, but this is not a requirement.

18.3. Custom SQL for create, update and delete

Hibernate can use custom SQL for create, update, and delete operations. The SQL can be overridden at the statementlevel or inidividual column level. This section describes statement overrides. For columns, see Section 5.6, “Columntransformers: read and write expressions”. Example 18.11, “Custom CRUD via annotations” shows how to define customSQL operatons using annotations.

Example 18.11. Custom CRUD via annotations

@Entity@Table(name="CHAOS")@SQLInsert( sql="INSERT INTO CHAOS(size, name, nickname, id) VALUES(?,upper(?),?,?)")@SQLUpdate( sql="UPDATE CHAOS SET size = ?, name = upper(?), nickname = ? WHERE id = ?")@SQLDelete( sql="DELETE CHAOS WHERE id = ?")@SQLDeleteAll( sql="DELETE CHAOS")@Loader(namedQuery = "chaos")@NamedNativeQuery(name="chaos", query="select id, size, name, lower( nickname ) as nickname from CHAOS where xml:id= ?", resultClass = Chaos.class)public class Chaos { @Id private Long id; private Long size; private String name; private String nickname;

@SQLInsert, @SQLUpdate, @SQLDelete, @SQLDeleteAll respectively override the INSERT, UPDATE, DELETE, and DELETE

all statement. The same can be achieved using Hibernate mapping files and the <sql-insert>, <sql-update> and <sql-delete>

nodes. This can be seen in Example 18.12, “Custom CRUD XML”.

Example 18.12. Custom CRUD XML

<class name="Person"> <id name="id"> <generator class="increment"/> </id> <property name="name" not-null="true"/> <sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-insert> <sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE xml:id=?</sql-update> <sql-delete>DELETE FROM PERSON WHERE xml:id=?</sql-delete></class>

If you expect to call a store procedure, be sure to set the callable attribute to true. In annotations as well as in xml.

To check that the execution happens correctly, Hibernate allows you to define one of those three strategies:

none: no check is performed: the store procedure is expected to fail upon issues

count: use of rowcount to check that the update is successful

param: like COUNT but using an output parameter rather that the standard mechanism

To define the result check style, use the check parameter which is again available in annoations as well as in xml.

You can use the exact same set of annotations respectively xml nodes to override the collection related statements -seeExample 18.13, “Overriding SQL statements for collections using annotations”.

Example 18.13. Overriding SQL statements for collections using annotations

@OneToMany@JoinColumn(name="chaos_fk")@SQLInsert( sql="UPDATE CASIMIR_PARTICULE SET chaos_fk = ? where id = ?")@SQLDelete( sql="UPDATE CASIMIR_PARTICULE SET chaos_fk = null where id = ?")private Set<CasimirParticle> particles = new HashSet<CasimirParticle>();

Tip

The parameter order is important and is defined by the order Hibernate handlesproperties. You can see the expected order by enabling debug logging for the

org.hibernate.persister.entity level. With this level enabled Hibernate will print out the static

SQL that is used to create, update, delete etc. entities. (To see the expected sequence,remember to not include your custom SQL through annotations or mapping files as thatwill override the Hibernate generated static sql)

Overriding SQL statements for secondary tables is also possible using @org.hibernate.annotations.Table and either (or all)

attributes sqlInsert, sqlUpdate, sqlDelete:

Example 18.14. Overriding SQL statements for secondary tables

@Entity@SecondaryTables({ @SecondaryTable(name = "`Cat nbr1`"), @SecondaryTable(name = "Cat2"})@org.hibernate.annotations.Tables( { @Table(appliesTo = "Cat", comment = "My cat table" ), @Table(appliesTo = "Cat2", foreignKey = @ForeignKey(name="FK_CAT2_CAT"), fetch = FetchMode.SELECT, sqlInsert=@SQLInsert(sql="insert into Cat2(storyPart2, id) values(upper(?), ?)") )} )public class Cat implements Serializable {

The previous example also shows that you can give a comment to a given table (primary or secondary): This comment willbe used for DDL generation.

Tip

The SQL is directly executed in your database, so you can use any dialect you like. Thiswill, however, reduce the portability of your mapping if you use database specific SQL.

Last but not least, stored procedures are in most cases required to return the number of rows inserted, updated anddeleted. Hibernate always registers the first statement parameter as a numeric output parameter for the CUD operations:

Example 18.15. Stored procedures and their return value

CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN VARCHAR2) RETURN NUMBER ISBEGIN

update PERSON set NAME = uname, where ID = uid;

return SQL%ROWCOUNT;

END updatePerson;

18.4. Custom SQL for loading

You can also declare your own SQL (or HQL) queries for entity loading. As with inserts, updates, and deletes, this can bedone at the individual column level as described in Section 5.6, “Column transformers: read and write expressions” or atthe statement level. Here is an example of a statement level override:

<sql-query name="person"> <return alias="pers" class="Person" lock-mode="upgrade"/> SELECT NAME AS {pers.name}, ID AS {pers.id} FROM PERSON WHERE xml:id=? FOR UPDATE</sql-query>

This is just a named query declaration, as discussed earlier. You can reference this named query in a class mapping:

<class name="Person"> <id name="id"> <generator class="increment"/> </id> <property name="name" not-null="true"/> <loader query-ref="person"/></class>

This even works with stored procedures.

You can even define a query for collection loading:

<set name="employments" inverse="true"> <key/> <one-to-many class="Employment"/> <loader query-ref="employments"/></set>

<sql-query name="employments"> <load-collection alias="emp" role="Person.employments"/> SELECT {emp.*} FROM EMPLOYMENT emp WHERE EMPLOYER = :id ORDER BY STARTDATE ASC, EMPLOYEE ASC</sql-query>

You can also define an entity loader that loads a collection by join fetching:

<sql-query name="person"> <return alias="pers" class="Person"/> <return-join alias="emp" property="pers.employments"/> SELECT NAME AS {pers.*}, {emp.*} FROM PERSON pers LEFT OUTER JOIN EMPLOYMENT emp ON pers.ID = emp.PERSON_ID WHERE xml:id=?</sql-query>

The annotation equivalent <loader> is the @Loader annotation as seen in Example 18.11, “Custom CRUD via

annotations”.

Chapter 19. Filtering data

Table of Contents


Hibernate provides an innovative new approach to handling data with "visibility" rules. A Hibernate filter is a global,named, parameterized filter that can be enabled or disabled for a particular Hibernate session.


Hibernate has the ability to pre-define filter criteria and attach those filters at both a class level and a collection level. Afilter criteria allows you to define a restriction clause similar to the existing "where" attribute available on the class andvarious collection elements. These filter conditions, however, can be parameterized. The application can then decide atruntime whether certain filters should be enabled and what their parameter values should be. Filters can be used likedatabase views, but they are parameterized inside the application.

Using annotatons filters are defined via @org.hibernate.annotations.FilterDef or @org.hibernate.annotations.FilterDefs. A filter

definition has a name() and an array of parameters(). A parameter will allow you to adjust the behavior of the filter at

runtime. Each parameter is defined by a @ParamDef which has a name and a type. You can also define a defaultCondition()

parameter for a given @FilterDef to set the default condition to use when none are defined in each individual @Filter.

@FilterDef(s) can be defined at the class or package level.

We now need to define the SQL filter clause applied to either the entity load or the collection load. @Filter is used and

placed either on the entity or the collection element. The connection between @FilterName and @Filter is a matching name.

Example 19.1. @FilterDef and @Filter annotations

@Entity@FilterDef(name="minLength", parameters=@ParamDef( name="minLength", type="integer" ) )@Filters( { @Filter(name="betweenLength", condition=":minLength <= length and :maxLength >= length"), @Filter(name="minLength", condition=":minLength <= length")} )public class Forest { ... }

When the collection use an association table as a relational representation, you might want to apply the filter condition tothe association table itself or to the target entity table. To apply the constraint on the target entity, use the regular @Filter

annotation. However, if you want to target the association table, use the @FilterJoinTable annotation.

Example 19.2. Using @FilterJoinTable for filterting on the association table

@OneToMany@JoinTable//filter on the target entity table@Filter(name="betweenLength", condition=":minLength <= length and :maxLength >= length")//filter on the association table@FilterJoinTable(name="security", condition=":userlevel >= requredLevel")public Set<Forest> getForests() { ... }

By default, Hibernate attempts to automatically determine all points within the @Filter SQL condition fragment that an alias

should be injected. To control the alias injection, set deduceAliasInjectionPoints to false within the @Filter. Injection points are

then marked using @SqlFragmentAlias annotations or within the SQL's condition fragment using {alias}.

In addition to allowing explicit alias control, deduceAliasInjectionPoints provides an out when Hibernate assumes an ANSI

SQL reserved keyword is a column and incorrectly aliases it.

Example 19.3. @Filter annotation, disabling deduceAliasInjectionPoints

@Entity@Table(name="T_TREE")@Filters({ @Filter(name="isTall", condition="{alias}.LENGTH >= 100", deduceAliasInjectionPoints = false), @Filter(name="isOak", condition="{t}.WOODTYPE like 'oak'", deduceAliasInjectionPoints = false, aliases={@SqlFragmentAlias(alias="t", table="T_TREE")})})public class Tree { ... }

Using Hibernate mapping files for defining filters the situtation is very similar. The filters must first be defined and thenattached to the appropriate mapping elements. To define a filter, use the <filter-def/> element within a <hibernate-mapping/>

element:

Example 19.4. Defining a filter definition via <filter-def>

<filter-def name="myFilter"> <filter-param name="myFilterParam" type="string"/></filter-def>

This filter can then be attached to a class or collection (or, to both or multiples of each at the same time):

Example 19.5. Attaching a filter to a class or collection using <filter>

<class name="myClass" ...> ... <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>

<set ...> <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/> </set> </class>

The methods on Session are: enableFilter(String filterName), getEnabledFilter(String filterName), and disableFilter(String filterName).

By default, filters are not enabled for a given session. Filters must be enabled through use of the Session.enableFilter()

method, which returns an instance of the Filter interface. If you used the simple filter defined above, it would look like this:

session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");

Methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hibernate.

The following is a full example, using temporal data with an effective record date pattern:

<filter-def name="effectiveDate"> <filter-param name="asOfDate" type="date"/></filter-def>

<class name="Employee" ...>... <many-to-one name="department" column="dept_id" class="Department"/> <property name="effectiveStartDate" type="date" column="eff_start_dt"/> <property name="effectiveEndDate" type="date" column="eff_end_dt"/>...  <filter name="effectiveDate" condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/></class>

<class name="Department" ...>... <set name="employees" lazy="true"> <key column="dept_id"/> <one-to-many class="Employee"/> <filter name="effectiveDate" condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/> </set></class>

In order to ensure that you are provided with currently effective records, enable the filter on the session prior to retrievingemployee data:

Session session = ...;session.enableFilter("effectiveDate").setParameter("asOfDate", new Date());List results = session.createQuery("from Employee as e where e.salary > :targetSalary") .setLong("targetSalary", new Long(1000000)) .list();

Even though a salary constraint was mentioned explicitly on the results in the above HQL, because of the enabled filter,the query will return only currently active employees who have a salary greater than one million dollars.

If you want to use filters with outer joining, either through HQL or load fetching, be careful of the direction of the conditionexpression. It is safest to set this up for left outer joining. Place the parameter first followed by the column name(s) after theoperator.

After being defined, a filter might be attached to multiple entities and/or collections each with its own condition. This can beproblematic when the conditions are the same each time. Using <filter-def/> allows you to definine a default condition,

either as an attribute or CDATA:

<filter-def name="myFilter" condition="abc > xyz">...</filter-def><filter-def name="myOtherFilter">abc=xyz</filter-def>

This default condition will be used whenever the filter is attached to something without specifying a condition. This meansyou can give a specific condition as part of the attachment of the filter that overrides the default condition in that particularcase.

Chapter 20. Improving performance

Table of Contents

20.1.1. Working with lazy associations20.1.2. Tuning fetch strategies20.1.3. Single-ended association proxies20.1.4. Initializing collections and proxies20.1.5. Using batch fetching20.1.6. Using subselect fetching20.1.7. Fetch profiles20.1.8. Using lazy property fetching


20.2.1. Cache mappings20.2.2. Strategy: read only20.2.3. Strategy: read/write20.2.4. Strategy: nonstrict read/write20.2.5. Strategy: transactional20.2.6. Cache-provider/concurrency-strategy compatibility

20.3. Managing the caches20.4. The Query Cache

20.4.1. Enabling query caching20.4.2. Query cache regions



20.5.2. Runtime instrument20.5.3. Buildtime instrument


20.6.1. Taxonomy20.6.2. Lists, maps, idbags and sets are the most efficient collections to update20.6.3. Bags and lists are the most efficient inverse collections20.6.4. One shot delete


20.7.1. Monitoring a SessionFactory20.7.2. Metrics


Hibernate uses a fetching strategy to retrieve associated objects if the application needs to navigate the association. Fetchstrategies can be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

Hibernate defines the following fetching strategies:

Join fetching: Hibernate retrieves the associated instance or collection in the same SELECT, using an OUTER JOIN.

Select fetching: a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable

lazy fetching by specifying lazy="false", this second select will only be executed when you access the association.

Subselect fetching: a second SELECT is used to retrieve the associated collections for all entities retrieved in a

previous query or fetch. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only

be executed when you access the association.

Batch fetching: an optimization strategy for select fetching. Hibernate retrieves a batch of entity instances or collectionsin a single SELECT by specifying a list of primary or foreign keys.

Hibernate also distinguishes between:

Immediate fetching: an association, collection or attribute is fetched immediately when the owner is loaded.

Lazy collection fetching: a collection is fetched when the application invokes an operation upon that collection. This isthe default for collections.

"Extra-lazy" collection fetching: individual elements of the collection are accessed from the database as needed.Hibernate tries not to fetch the whole collection into memory unless absolutely needed. It is suitable for largecollections.

Proxy fetching: a single-valued association is fetched when a method other than the identifier getter is invoked uponthe associated object.

"No-proxy" fetching: a single-valued association is fetched when the instance variable is accessed. Compared toproxy fetching, this approach is less lazy; the association is fetched even when only the identifier is accessed. It is alsomore transparent, since no proxy is visible to the application. This approach requires buildtime bytecodeinstrumentation and is rarely necessary.

Lazy attribute fetching: an attribute or single valued association is fetched when the instance variable is accessed.This approach requires buildtime bytecode instrumentation and is rarely necessary.

We have two orthogonal notions here: when is the association fetched and how is it fetched. It is important that you do notconfuse them. We use fetch to tune performance. We can use lazy to define a contract for what data is always available in

any detached instance of a particular class.

20.1.1. Working with lazy associations

By default, Hibernate uses lazy select fetching for collections and lazy proxy fetching for single-valued associations. Thesedefaults make sense for most associations in the majority of applications.

If you set hibernate.default_batch_fetch_size, Hibernate will use the batch fetch optimization for lazy fetching. This

optimization can also be enabled at a more granular level.

Please be aware that access to a lazy association outside of the context of an open Hibernate session will result in anexception. For example:

s = sessions.openSession();Transaction tx = s.beginTransaction(); User u = (User) s.createQuery("from User u where u.name=:userName") .setString("userName", userName).uniqueResult();Map permissions = u.getPermissions();

tx.commit();s.close();

Integer accessLevel = (Integer) permissions.get("accounts"); // Error!

Since the permissions collection was not initialized when the Session was closed, the collection will not be able to load its

state. Hibernate does not support lazy initialization for detached objects. This can be fixed by moving the code that readsfrom the collection to just before the transaction is committed.

Alternatively, you can use a non-lazy collection or association, by specifying lazy="false" for the association mapping.

However, it is intended that lazy initialization be used for almost all collections and associations. If you define too manynon-lazy associations in your object model, Hibernate will fetch the entire database into memory in every transaction.

On the other hand, you can use join fetching, which is non-lazy by nature, instead of select fetching in a particulartransaction. We will now explain how to customize the fetching strategy. In Hibernate, the mechanisms for choosing a fetchstrategy are identical for single-valued associations and collections.

20.1.2. Tuning fetch strategies

Select fetching (the default) is extremely vulnerable to N+1 selects problems, so we might want to enable join fetching inthe mapping document:

<set name="permissions" fetch="join"> <key column="userId"/> <one-to-many class="Permission"/></set

<many-to-one name="mother" class="Cat" fetch="join"/>

The fetch strategy defined in the mapping document affects:

retrieval via get() or load()

retrieval that happens implicitly when an association is navigated

Criteria queries

HQL queries if subselect fetching is used

Irrespective of the fetching strategy you use, the defined non-lazy graph is guaranteed to be loaded into memory. Thismight, however, result in several immediate selects being used to execute a particular HQL query.

Usually, the mapping document is not used to customize fetching. Instead, we keep the default behavior, and override itfor a particular transaction, using left join fetch in HQL. This tells Hibernate to fetch the association eagerly in the first select,

using an outer join. In the Criteria query API, you would use setFetchMode(FetchMode.JOIN).

If you want to change the fetching strategy used by get() or load(), you can use a Criteria query. For example:

User user = (User) session.createCriteria(User.class) .setFetchMode("permissions", FetchMode.JOIN) .add( Restrictions.idEq(userId) ) .uniqueResult();

This is Hibernate's equivalent of what some ORM solutions call a "fetch plan".

A completely different approach to problems with N+1 selects is to use the second-level cache.

20.1.3. Single-ended association proxies

Lazy fetching for collections is implemented using Hibernate's own implementation of persistent collections. However, adifferent mechanism is needed for lazy behavior in single-ended associations. The target entity of the association must beproxied. Hibernate implements lazy initializing proxies for persistent objects using runtime bytecode enhancement whichis accessed via the bytecode provider.

At startup, Hibernate generates proxies by default for all persistent classes and uses them to enable lazy fetching ofmany-to-one and one-to-one associations.

The mapping file may declare an interface to use as the proxy interface for that class, with the proxy attribute. By default,

Hibernate uses a subclass of the class. The proxied class must implement a default constructor with at least packagevisibility. This constructor is recommended for all persistent classes.

There are potential problems to note when extending this approach to polymorphic classes.For example:

<class name="Cat" proxy="Cat"> ...... <subclass name="DomesticCat"> ..... </subclass></class>

Firstly, instances of Cat will never be castable to DomesticCat, even if the underlying instance is an instance of DomesticCat:

Cat cat = (Cat) session.load(Cat.class, id); // instantiate a proxy (does not hit the db)if ( cat.isDomesticCat() ) { // hit the db to initialize the proxy DomesticCat dc = (DomesticCat) cat; // Error! ....}

Secondly, it is possible to break proxy ==:

Cat cat = (Cat) session.load(Cat.class, id); // instantiate a Cat proxyDomesticCat dc = (DomesticCat) session.load(DomesticCat.class, id); // acquire new DomesticCat proxy!System.out.println(cat==dc); // false

However, the situation is not quite as bad as it looks. Even though we now have two references to different proxy objects,the underlying instance will still be the same object:

cat.setWeight(11.0); // hit the db to initialize the proxySystem.out.println( dc.getWeight() ); // 11.0

Third, you cannot use a bytecode provider generated proxy for a final class or a class with any final methods.

Finally, if your persistent object acquires any resources upon instantiation (e.g. in initializers or default constructor), thenthose resources will also be acquired by the proxy. The proxy class is an actual subclass of the persistent class.

These problems are all due to fundamental limitations in Java's single inheritance model. To avoid these problems yourpersistent classes must each implement an interface that declares its business methods. You should specify theseinterfaces in the mapping file where CatImpl implements the interface Cat and DomesticCatImpl implements the interface

DomesticCat. For example:

<class name="CatImpl" proxy="Cat"> ...... <subclass name="DomesticCatImpl" proxy="DomesticCat"> ..... </subclass></class>

Then proxies for instances of Cat and DomesticCat can be returned by load() or iterate().

Cat cat = (Cat) session.load(CatImpl.class, catid);Iterator iter = session.createQuery("from CatImpl as cat where cat.name='fritz'").iterate();Cat fritz = (Cat) iter.next();

Note

list() does not usually return proxies.

Relationships are also lazily initialized. This means you must declare any properties to be of type Cat, not CatImpl.

Certain operations do not require proxy initialization:

equals(): if the persistent class does not override equals()

hashCode(): if the persistent class does not override hashCode()

The identifier getter method

Hibernate will detect persistent classes that override equals() or hashCode().

By choosing lazy="no-proxy" instead of the default lazy="proxy", you can avoid problems associated with typecasting.

However, buildtime bytecode instrumentation is required, and all operations will result in immediate proxy initialization.

20.1.4. Initializing collections and proxies

A LazyInitializationException will be thrown by Hibernate if an uninitialized collection or proxy is accessed outside of the

scope of the Session, i.e., when the entity owning the collection or having the reference to the proxy is in the detached

state.

Sometimes a proxy or collection needs to be initialized before closing the Session. You can force initialization by calling

cat.getSex() or cat.getKittens().size(), for example. However, this can be confusing to readers of the code and it is not

convenient for generic code.

The static methods Hibernate.initialize() and Hibernate.isInitialized(), provide the application with a convenient way of working

with lazily initialized collections or proxies. Hibernate.initialize(cat) will force the initialization of a proxy, cat, as long as its

Session is still open. Hibernate.initialize( cat.getKittens() ) has a similar effect for the collection of kittens.

Another option is to keep the Session open until all required collections and proxies have been loaded. In some

application architectures, particularly where the code that accesses data using Hibernate, and the code that uses it are indifferent application layers or different physical processes, it can be a problem to ensure that the Session is open when a

collection is initialized. There are two basic ways to deal with this issue:

In a web-based application, a servlet filter can be used to close the Session only at the end of a user request, once the

rendering of the view is complete (the Open Session in View pattern). Of course, this places heavy demands on thecorrectness of the exception handling of your application infrastructure. It is vitally important that the Session is closed

and the transaction ended before returning to the user, even when an exception occurs during rendering of the view.See the Hibernate Wiki for examples of this "Open Session in View" pattern.

In an application with a separate business tier, the business logic must "prepare" all collections that the web tier needsbefore returning. This means that the business tier should load all the data and return all the data already initialized tothe presentation/web tier that is required for a particular use case. Usually, the application calls Hibernate.initialize() for

each collection that will be needed in the web tier (this call must occur before the session is closed) or retrieves thecollection eagerly using a Hibernate query with a FETCH clause or a FetchMode.JOIN in Criteria. This is usually easier if

you adopt the Command pattern instead of a Session Facade.

You can also attach a previously loaded object to a new Session with merge() or lock() before accessing uninitialized

collections or other proxies. Hibernate does not, and certainly should not, do this automatically since it wouldintroduce impromptu transaction semantics.

Sometimes you do not want to initialize a large collection, but still need some information about it, like its size, for example,or a subset of the data.

You can use a collection filter to get the size of a collection without initializing it:

( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()

The createFilter() method is also used to efficiently retrieve subsets of a collection without needing to initialize the whole

collection:

s.createFilter( lazyCollection, "").setFirstResult(0).setMaxResults(10).list();

20.1.5. Using batch fetching

Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is accessed. Batch fetching is anoptimization of the lazy select fetching strategy. There are two ways you can configure batch fetching: on the class leveland the collection level.

Batch fetching for classes/entities is easier to understand. Consider the following example: at runtime you have 25 Cat

instances loaded in a Session, and each Cat has a reference to its owner, a Person. The Person class is mapped with a

proxy, lazy="true". If you now iterate through all cats and call getOwner() on each, Hibernate will, by default, execute 25

SELECT statements to retrieve the proxied owners. You can tune this behavior by specifying a batch-size in the mapping of

Person:

<class name="Person" batch-size="10">...</class>

With this batch-size specified, Hibernate will now execute queries on demand when need to access the uninitialized proxy,

as above, but the difference is that instead of querying the exactly proxy entity that being accessed, it will query morePerson's owner at once, so, when accessing other person's owner, it may already been initialized by this batch fetch withonly a few ( much less than 25) queries will be executed.

This behavior is controlled by the batch-size and batch fetch style configuration. The batch fetch style configuration (

hibernate.batch_fetch_style ) is a new performance improvement since 4.2.0, there are 3 different strategies provided, which

is legacy, padded and dynamic.

LEGACY

The legacy algorithm where we keep a set of pre-built batch sizes based onorg.hibernate.internal.util.collections.ArrayHelper#getBatchSizes. Batches are performed using the next-smaller pre-built

batch size from the number of existing batchable identifiers.

In the above example, with a batch-size setting of 25 the pre-built batch sizes would be [25, 12, 10, 9, 8, 7, .., 1].

And since there are 25 persons' owner to be initialized, then only one query will be executed using these 25 owners'identifier.

But in another case, suppose there are only 24 persons, there will be 3 queries (12, 10, 2) will be executed to gothrough all person's owner, and the query will looks like :

select * from owner where id in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) select * from owner where id in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?) select * from owner where id in (?, ?)

PADDED

This is kind of similar with the legacy algorithm, it uses the pre-build batch sizes based on sameorg.hibernate.internal.util.collections.ArrayHelper#getBatchSizes. The difference is that here hibernate will use the next-

bigger batch size and pads the extra identifier placeholders.

So, using the same example above, initializing 25 persons the query would be same as above, only 1 query will beexecuted to batch query all the owners.

However, the attempt to batch load 24 owners would result just a single batch of size 25, the identifiers to load wouldbe "padded" (aka, repeated) to make up the difference.

DYNAMIC

Dynamically builds its SQL based on the actual number of available ids. Does still limit to the batch-size defined on theentity.

You can also enable batch fetching of collections. For example, if each Person has a lazy collection of Cats, and 10

persons are currently loaded in the Session, iterating through all persons will generate 10 SELECTs, one for every call to

getCats(). If you enable batch fetching for the cats collection in the mapping of Person, Hibernate can pre-fetch collections:

<class name="Person"> <set name="cats" batch-size="3"> ... </set></class>

For example, with a batch-size of 3 and using legacy batch style, Hibernate will load 3, 3, 3, 1 collections in four SELECTs.

Again, the value of the attribute depends on the expected number of uninitialized collections in a particular Session.

Batch fetching of collections is particularly useful if you have a nested tree of items, i.e. the typical bill-of-materials pattern.However, a nested set or a materialized path might be a better option for read-mostly trees.

20.1.6. Using subselect fetching

If one lazy collection or single-valued proxy has to be fetched, Hibernate will load all of them, re-running the original queryin a subselect. This works in the same way as batch-fetching but without the piecemeal loading.

20.1.7. Fetch profiles

Another way to affect the fetching strategy for loading associated objects is through something called a fetch profile, whichis a named configuration associated with the org.hibernate.SessionFactory but enabled, by name, on the

org.hibernate.Session. Once enabled on a org.hibernate.Session, the fetch profile will be in affect for that org.hibernate.Session

until it is explicitly disabled.

So what does that mean? Well lets explain that by way of an example which show the different available approaches toconfigure a fetch profile:

Example 20.1. Specifying a fetch profile using @FetchProfile

@Entity@FetchProfile(name = "customer-with-orders", fetchOverrides = { @FetchProfile.FetchOverride(entity = Customer.class, association = "orders", mode = FetchMode.JOIN)})public class Customer { @Id @GeneratedValue private long id;

private String name;

private long customerNumber;

@OneToMany private Set<Order> orders;

// standard getter/setter ...}

Example 20.2. Specifying a fetch profile using <fetch-profile> outside <class> node

<hibernate-mapping> <class name="Customer"> ... <set name="orders" inverse="true"> <key column="cust_id"/> <one-to-many class="Order"/> </set> </class> <class name="Order"> ... </class> <fetch-profile name="customer-with-orders"> <fetch entity="Customer" association="orders" style="join"/> </fetch-profile></hibernate-mapping>

Example 20.3. Specifying a fetch profile using <fetch-profile> inside <class> node

<hibernate-mapping> <class name="Customer"> ... <set name="orders" inverse="true"> <key column="cust_id"/> <one-to-many class="Order"/> </set> <fetch-profile name="customer-with-orders"> <fetch association="orders" style="join"/> </fetch-profile> </class> <class name="Order"> ... </class></hibernate-mapping>

Now normally when you get a reference to a particular customer, that customer's set of orders will be lazy meaning we willnot yet have loaded those orders from the database. Normally this is a good thing. Now lets say that you have a certainuse case where it is more efficient to load the customer and their orders together. One way certainly is to use "dynamicfetching" strategies via an HQL or criteria queries. But another option is to use a fetch profile to achieve that. The followingcode will load both the customer andtheir orders:

Example 20.4. Activating a fetch profile for a given Session

Session session = ...;session.enableFetchProfile( "customer-with-orders" ); // name matches from mappingCustomer customer = (Customer) session.get( Customer.class, customerId );

Note

@FetchProfile definitions are global and it does not matter on which class you place them.

You can place the @FetchProfile annotation either onto a class or package (package-

info.java). In order to define multiple fetch profiles for the same class or package@FetchProfiles can be used.

Currently only join style fetch profiles are supported, but they plan is to support additional styles. See HHH-3414 fordetails.

http://opensource.atlassian.com/projects/hibernate/browse/HHH-3414

20.1.8. Using lazy property fetching

Hibernate supports the lazy fetching of individual properties. This optimization technique is also known as fetch groups.Please note that this is mostly a marketing feature; optimizing row reads is much more important than optimization ofcolumn reads. However, only loading some properties of a class could be useful in extreme cases. For example, whenlegacy tables have hundreds of columns and the data model cannot be improved.

To enable lazy property loading, set the lazy attribute on your particular property mappings:

<class name="Document"> <id name="id"> <generator class="native"/> </id> <property name="name" not-null="true" length="50"/> <property name="summary" not-null="true" length="200" lazy="true"/> <property name="text" not-null="true" length="2000" lazy="true"/></class>

Lazy property loading requires buildtime bytecode instrumentation. If your persistent classes are not enhanced, Hibernatewill ignore lazy property settings and return to immediate fetching.

For bytecode instrumentation, use the following Ant task:

<target name="instrument" depends="compile"> <taskdef name="instrument" classname="org.hibernate.tool.instrument.InstrumentTask"> <classpath path="${jar.path}"/> <classpath path="${classes.dir}"/> <classpath refxml:id="lib.class.path"/> </taskdef>

<instrument verbose="true"> <fileset dir="${testclasses.dir}/org/hibernate/auction/model"> <include name="*.class"/> </fileset> </instrument></target>

A different way of avoiding unnecessary column reads, at least for read-only transactions, is to use the projection featuresof HQL or Criteria queries. This avoids the need for buildtime bytecode processing and is certainly a preferred solution.

You can force the usual eager fetching of properties using fetch all properties in HQL.


A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a cluster or JVM-level

(SessionFactory-level) cache on a class-by-class and collection-by-collection basis. You can even plug in a clustered

cache. Be aware that caches are not aware of changes made to the persistent store by another application. They can,however, be configured to regularly expire cached data.

You have the option to tell Hibernate which caching implementation to use by specifying the name of a class thatimplements org.hibernate.cache.spi.CacheProvider using the property hibernate.cache.provider_class. Hibernate is bundled with

a number of built-in integrations with the open-source cache providers that are listed in Table 20.1, “Cache Providers”.You can also implement your own and plug it in as outlined above. Note that versions prior to Hibernate 3.2 use EhCacheas the default cache provider.

Table 20.1. Cache Providers

Cache Provider class TypeClusterSafe

QueryCacheSupported

ConcurrentHashMap(only for testingpurpose, in hibernate-testing module)

org.hibernate.testing.cache.CachingRegionFactory memory yes

EHCache org.hibernate.cache.ehcache.EhCacheRegionFactory

memory,disk,transactional,clustered

yes yes

Infinispan org.hibernate.cache.infinispan.InfinispanRegionFactoryclustered (ipmulticast),transactional

yes(replicationorinvalidation)

yes (clocksync req.)

20.2.1. Cache mappings

As we have done in previous chapters we are looking at the two different possibiltites to configure caching. Firstconfiguration via annotations and then via Hibernate mapping files.

By default, entities are not part of the second level cache and we recommend you to stick to this setting. However, you canoverride this by setting the shared-cache-mode element in your persistence.xml file or by using the

javax.persistence.sharedCache.mode property in your configuration. The following values are possible:

ENABLE_SELECTIVE (Default and recommended value): entities are not cached unless explicitly marked as cacheable.

DISABLE_SELECTIVE: entities are cached unless explicitly marked as not cacheable.

ALL: all entities are always cached even if marked as non cacheable.

NONE: no entity are cached even if marked as cacheable. This option can make sense to disable second-level cache

altogether.

The cache concurrency strategy used by default can be set globaly via thehibernate.cache.default_cache_concurrency_strategy configuration property. The values for this property are:

read-only

read-write

nonstrict-read-write

transactional

Note

It is recommended to define the cache concurrency strategy per entity rather than using aglobal one. Use the @org.hibernate.annotations.Cache annotation for that.

Example 20.5. Definition of cache concurrency strategy via @Cache

@Entity @Cacheable@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)public class Forest { ... }

Hibernate also let's you cache the content of a collection or the identifiers if the collection contains other entities. Use the@Cache annotation on the collection property.

Example 20.6. Caching collections using annotations

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)@JoinColumn(name="CUST_ID")@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)public SortedSet<Ticket> getTickets() { return tickets;}

Example 20.7, “@Cache annotation with attributes”show s the @org.hibernate.annotations.Cache annotations with its

attributes. It allows you to define the caching strategy and region of a given second level cache.

Example 20.7. @Cache annotation with attributes

@Cache( CacheConcurrencyStrategy usage(); String region() default ""; String include() default "all"; )

usage: the given cache concurrency strategy (NONE, READ_ONLY, NONSTRICT_READ_WRITE,READ_WRITE, TRANSACTIONAL)

region (optional): the cache region (default to the fqcn of the class or the fq role name of the collection)

include (optional): all to include all properties, non-lazy to only include non lazy properties (default all).

Let's now take a look at Hibernate mapping files. There the <cache> element of a class or collection mapping is used to

configure the second level cache. Looking at Example 20.8, “The Hibernate <cache> mapping element” the parallels to

anotations is obvious.

Example 20.8. The Hibernate <cache> mapping element

<cache usage="transactional|read-write|nonstrict-read-write|read-only" region="RegionName" include="all|non-lazy" />

usage (required) specifies the caching strategy: transactional, read-write, nonstrict-read-write or read-only

region (optional: defaults to the class or collection role name): specifies the name of the second level cache

region

include (optional: defaults to all) non-lazy: specifies that properties of the entity mapped with lazy="true" cannot be

cached when attribute-level lazy fetching is enabled

Alternatively to <cache>, you can use <class-cache> and <collection-cache> elements in hibernate.cfg.xml.

Let's now have a closer look at the different usage strategies

20.2.2. Strategy: read only

If your application needs to read, but not modify, instances of a persistent class, a read-only cache can be used. This is the

simplest and optimal performing strategy. It is even safe for use in a cluster.

20.2.3. Strategy: read/write

If the application needs to update data, a read-write cache might be appropriate. This cache strategy should never be used

if serializable transaction isolation level is required. If the cache is used in a JTA environment, you must specify theproperty hibernate.transaction.manager_lookup_class and naming a strategy for obtaining the JTA TransactionManager. In other

environments, you should ensure that the transaction is completed when Session.close() or Session.disconnect() is called. If

you want to use this strategy in a cluster, you should ensure that the underlying cache implementation supports locking.The built-in cache providers do not support locking.

20.2.4. Strategy: nonstrict read/write

If the application only occasionally needs to update data (i.e. if it is extremely unlikely that two transactions would try toupdate the same item simultaneously), and strict transaction isolation is not required, a nonstrict-read-write cache might be

appropriate. If the cache is used in a JTA environment, you must specify hibernate.transaction.manager_lookup_class. In other

environments, you should ensure that the transaction is completed when Session.close() or Session.disconnect() is called.

20.2.5. Strategy: transactional

The transactional cache strategy provides support for fully transactional cache providers such as JBoss TreeCache. Such a

cache can only be used in a JTA environment and you must specify hibernate.transaction.manager_lookup_class.

20.2.6. Cache-provider/concurrency-strategy compatibility

Important

None of the cache providers support all of the cache concurrency strategies.

The following table shows which providers are compatible with which concurrency strategies.

Table 20.2. Cache Concurrency Strategy Support

Cache read-only nonstrict-read-write read-write transactional

ConcurrentHashMap (not intended for production use) yes yes yes

EHCache yes yes yes yes

Infinispan yes yes

20.3. Managing the caches

Whenever you pass an object to save(), update() or saveOrUpdate(), and whenever you retrieve an object using load(), get(),

list(), iterate() or scroll(), that object is added to the internal cache of the Session.

When flush() is subsequently called, the state of that object will be synchronized with the database. If you do not want this

synchronization to occur, or if you are processing a huge number of objects and need to manage memory efficiently, theevict() method can be used to remove the object and its collections from the first-level cache.

Example 20.9. Explcitly evicting a cached instance from the first level cache using Session.evict()

ScrollableResult cats = sess.createQuery("from Cat as cat").scroll(); //a huge result setwhile ( cats.next() ) { Cat cat = (Cat) cats.get(0); doSomethingWithACat(cat); sess.evict(cat);}

The Session also provides a contains() method to determine if an instance belongs to the session cache.

To evict all objects from the session cache, call Session.clear()

For the second-level cache, there are methods defined on SessionFactory for evicting the cached state of an instance,

entire class, collection instance or entire collection role.

Example 20.10. Second-level cache eviction via SessionFactoty.evict() and SessionFacyory.evictCollection()

sessionFactory.evict(Cat.class, catId); //evict a particular CatsessionFactory.evict(Cat.class); //evict all CatssessionFactory.evictCollection("Cat.kittens", catId); //evict a particular collection of kittenssessionFactory.evictCollection("Cat.kittens"); //evict all kitten collections

The CacheMode controls how a particular session interacts with the second-level cache:

CacheMode.NORMAL: will read items from and write items to the second-level cache

CacheMode.GET: will read items from the second-level cache. Do not write to the second-level cache except when

updating data

CacheMode.PUT: will write items to the second-level cache. Do not read from the second-level cache

CacheMode.REFRESH: will write items to the second-level cache. Do not read from the second-level cache. Bypass the

effect of hibernate.cache.use_minimal_puts forcing a refresh of the second-level cache for all items read from the

database

To browse the contents of a second-level or query cache region, use the Statistics API:

Example 20.11. Browsing the second-level cache entries via the Statistics API

Map cacheEntries = sessionFactory.getStatistics() .getSecondLevelCacheStatistics(regionName) .getEntries();

You will need to enable statistics and, optionally, force Hibernate to keep the cache entries in a more readable format:

Example 20.12. Enabling Hibernate statistics

hibernate.generate_statistics truehibernate.cache.use_structured_entries true

20.4. The Query Cache

Query result sets can also be cached. This is only useful for queries that are run frequently with the same parameters.

20.4.1. Enabling query caching

Caching of query results introduces some overhead in terms of your applications normal transactional processing. Forexample, if you cache results of a query against Person Hibernate will need to keep track of when those results should beinvalidated because changes have been committed against Person. That, coupled with the fact that most applicationssimply gain no benefit from caching query results, leads Hibernate to disable caching of query results by default. To usequery caching, you will first need to enable the query cache:

hibernate.cache.use_query_cache true

This setting creates two new cache regions:

org.hibernate.cache.internal.StandardQueryCache, holding the cached query results

org.hibernate.cache.spi.UpdateTimestampsCache, holding timestamps of the most recent updates to queryable tables.

These are used to validate the results as they are served from the query cache.

Important

If you configure your underlying cache implementation to use expiry or timeouts is veryimportant that the cache timeout of the underlying cache region for theUpdateTimestampsCache be set to a higher value than the timeouts of any of the querycaches. In fact, we recommend that the the UpdateTimestampsCache region not beconfigured for expiry at all. Note, in particular, that an LRU cache expiry policy is neverappropriate.

As mentioned above, most queries do not benefit from caching or their results. So by default, individual queries are notcached even after enabling query caching. To enable results caching for a particular query, callorg.hibernate.Query.setCacheable(true). This call allows the query to look for existing cache results or add its results to the

cache when it is executed.

Note

The query cache does not cache the state of the actual entities in the cache; it cachesonly identifier values and results of value type. For this reaso, the query cache shouldalways be used in conjunction with the second-level cache for those entities expected tobe cached as part of a query result cache (just as with collection caching).

20.4.2. Query cache regions

If you require fine-grained control over query cache expiration policies, you can specify a named cache region for aparticular query by calling Query.setCacheRegion().

List blogs = sess.createQuery("from Blog blog where blog.blogger = :blogger") .setEntity("blogger", blogger) .setMaxResults(15) .setCacheable(true) .setCacheRegion("frontpages") .list();

If you want to force the query cache to refresh one of its regions (disregard any cached results it finds there) you can useorg.hibernate.Query.setCacheMode(CacheMode.REFRESH). In conjunction with the region you have defined for the given

query, Hibernate will selectively force the results cached in that particular region to be refreshed. This is particularly usefulin cases where underlying data may have been updated via a separate process and is a far more efficient alternative tobulk eviction of the region via org.hibernate.SessionFactory.evictQueries().


Hibernate internally needs an entry ( org.hibernate.engine.spi.EntityEntry ) to tell the current state of an object with respect to

its persistent state, when the object is associated with a Session. However, maintaining this association was kind of heavy

operation due to lots of other rules must by applied, since 4.2.0, there is a new improvement designed for this purpose,which will reduce session-related memory and CPU overloads.

Basically, the idea is, instead of having a customized ( kind of heavy and which was usually identified as hotspot ) map todo the look up, we change it to

EntityEntry entry = (ManagedEntity)entity.$$_hibernate_getEntityEntry();

There are three ways to get benefits from this new improvement:


An entity can choose to implement this interface by itself, then it is the entity's responsibility to maintain the bi-associationthat essentially provides access to information about an instance's association to a Session/EntityManager. More infoabout org.hibernate.engine.spi.ManagedEntity please find from its javadoc.

20.5.2. Runtime instrument

Sometimes, you probably don't want to implement an intrusive interface, maybe due to portable concern, which is fine andHibernate will take care of this internally with a wrapper class which implements that interface, and also an internal cachethat maps this entity instance and the wrapper together.

Obviously, this is the easiest way to choose, since it doesn't require any change of the project source code, but it also costmore memory and CUP usage, comparing to the first one.

20.5.3. Buildtime instrument

Besides above two approaches, Hibernate also provides you the third choice, which is an ANT task (org.hibernate.tool.enhance.EnhancementTask) that can be used at build time to instrument your entity classes with the required

interfaces and methods. Meanwhile, the maven and gradle plugin are still under development and will be available soon.


In the previous sections we have covered collections and their applications. In this section we explore some more issuesin relation to collections at runtime.

20.6.1. Taxonomy

Hibernate defines three basic kinds of collections:

collections of values

one-to-many associations

many-to-many associations

This classification distinguishes the various table and foreign key relationships but does not tell us quite everything weneed to know about the relational model. To fully understand the relational structure and performance characteristics, wemust also consider the structure of the primary key that is used by Hibernate to update or delete collection rows. Thissuggests the following classification:

indexed collections

sets

bags

All indexed collections (maps, lists, and arrays) have a primary key consisting of the <key> and <index> columns. In this

case, collection updates are extremely efficient. The primary key can be efficiently indexed and a particular row can beefficiently located when Hibernate tries to update or delete it.

Sets have a primary key consisting of <key> and element columns. This can be less efficient for some types of collection

element, particularly composite elements or large text or binary fields, as the database may not be able to index a complex

primary key as efficiently. However, for one-to-many or many-to-many associations, particularly in the case of syntheticidentifiers, it is likely to be just as efficient. If you want SchemaExport to actually create the primary key of a <set>, you must

declare all columns as not-null="true".

<idbag> mappings define a surrogate key, so they are efficient to update. In fact, they are the best case.

Bags are the worst case since they permit duplicate element values and, as they have no index column, no primary keycan be defined. Hibernate has no way of distinguishing between duplicate rows. Hibernate resolves this problem bycompletely removing in a single DELETE and recreating the collection whenever it changes. This can be inefficient.

For a one-to-many association, the "primary key" may not be the physical primary key of the database table. Even in thiscase, the above classification is still useful. It reflects how Hibernate "locates" individual rows of the collection.

20.6.2. Lists, maps, idbags and sets are the most efficient collections to update

From the discussion above, it should be clear that indexed collections and sets allow the most efficient operation in termsof adding, removing and updating elements.

There is, arguably, one more advantage that indexed collections have over sets for many-to-many associations orcollections of values. Because of the structure of a Set, Hibernate does not UPDATE a row when an element is "changed".

Changes to a Set always work via INSERT and DELETE of individual rows. Once again, this consideration does not apply to

one-to-many associations.

After observing that arrays cannot be lazy, you can conclude that lists, maps and idbags are the most performant (non-inverse) collection types, with sets not far behind. You can expect sets to be the most common kind of collection inHibernate applications. This is because the "set" semantics are most natural in the relational model.

However, in well-designed Hibernate domain models, most collections are in fact one-to-many associations withinverse="true". For these associations, the update is handled by the many-to-one end of the association, and so

considerations of collection update performance simply do not apply.

20.6.3. Bags and lists are the most efficient inverse collections

There is a particular case, however, in which bags, and also lists, are much more performant than sets. For a collectionwith inverse="true", the standard bidirectional one-to-many relationship idiom, for example, we can add elements to a bag

or list without needing to initialize (fetch) the bag elements. This is because, unlike a set, Collection.add() or Collection.addAll()

must always return true for a bag or List. This can make the following common code much faster:

Parent p = (Parent) sess.load(Parent.class, id);Child c = new Child();c.setParent(p);p.getChildren().add(c); //no need to fetch the collection!sess.flush();

20.6.4. One shot delete

Deleting collection elements one by one can sometimes be extremely inefficient. Hibernate knows not to do that in thecase of an newly-empty collection (if you called list.clear(), for example). In this case, Hibernate will issue a single DELETE.

Suppose you added a single element to a collection of size twenty and then remove two elements. Hibernate will issueone INSERT statement and two DELETE statements, unless the collection is a bag. This is certainly desirable.

However, suppose that we remove eighteen elements, leaving two and then add thee new elements. There are twopossible ways to proceed

delete eighteen rows one by one and then insert three rows

remove the whole collection in one SQL DELETE and insert all five current elements one by one

Hibernate cannot know that the second option is probably quicker. It would probably be undesirable for Hibernate to bethat intuitive as such behavior might confuse database triggers, etc.

Fortunately, you can force this behavior (i.e. the second strategy) at any time by discarding (i.e. dereferencing) the originalcollection and returning a newly instantiated collection with all the current elements.

One-shot-delete does not apply to collections mapped inverse="true".


Optimization is not much use without monitoring and access to performance numbers. Hibernate provides a full range offigures about its internal operations. Statistics in Hibernate are available per SessionFactory.

20.7.1. Monitoring a SessionFactory

You can access SessionFactory metrics in two ways. Your first option is to call sessionFactory.getStatistics() and read or

display the Statistics yourself.

Hibernate can also use JMX to publish metrics if you enable the StatisticsService MBean. You can enable a single MBean

for all your SessionFactory or one per factory. See the following code for minimalistic configuration examples:

// MBean service registration for a specific SessionFactoryHashtable tb = new Hashtable();tb.put("type", "statistics");tb.put("sessionFactory", "myFinancialApp");ObjectName on = new ObjectName("hibernate", tb); // MBean object name

StatisticsService stats = new StatisticsService(); // MBean implementationstats.setSessionFactory(sessionFactory); // Bind the stats to a SessionFactoryserver.registerMBean(stats, on); // Register the Mbean on the server

// MBean service registration for all SessionFactory'sHashtable tb = new Hashtable();tb.put("type", "statistics");tb.put("sessionFactory", "all");ObjectName on = new ObjectName("hibernate", tb); // MBean object name

StatisticsService stats = new StatisticsService(); // MBean implementationserver.registerMBean(stats, on); // Register the MBean on the server

You can activate and deactivate the monitoring for a SessionFactory:

at configuration time, set hibernate.generate_statistics to false

at runtime: sf.getStatistics().setStatisticsEnabled(true) or hibernateStatsBean.setStatisticsEnabled(true)

Statistics can be reset programmatically using the clear() method. A summary can be sent to a logger (info level) using the

logSummary() method.

20.7.2. Metrics

Hibernate provides a number of metrics, from basic information to more specialized information that is only relevant incertain scenarios. All available counters are described in the Statistics interface API, in three categories:

Metrics related to the general Session usage, such as number of open sessions, retrieved JDBC connections, etc.

Metrics related to the entities, collections, queries, and caches as a whole (aka global metrics).

Detailed metrics related to a particular entity, collection, query or cache region.

For example, you can check the cache hit, miss, and put ratio of entities, collections and queries, and the average time aquery needs. Be aware that the number of milliseconds is subject to approximation in Java. Hibernate is tied to the JVMprecision and on some platforms this might only be accurate to 10 seconds.

Simple getters are used to access the global metrics (i.e. not tied to a particular entity, collection, cache region, etc.). Youcan access the metrics of a particular entity, collection or cache region through its name, and through its HQL or SQLrepresentation for queries. Please refer to the Statistics, EntityStatistics, CollectionStatistics, SecondLevelCacheStatistics, and

QueryStatistics API Javadoc for more information. The following code is a simple example:

Statistics stats = HibernateUtil.sessionFactory.getStatistics();

double queryCacheHitCount = stats.getQueryCacheHitCount();double queryCacheMissCount = stats.getQueryCacheMissCount();double queryCacheHitRatio = queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);

log.info("Query Hit ratio:" + queryCacheHitRatio);

EntityStatistics entityStats = stats.getEntityStatistics( Cat.class.getName() );long changes = entityStats.getInsertCount() + entityStats.getUpdateCount() + entityStats.getDeleteCount();log.info(Cat.class.getName() + " changed " + changes + "times" );

You can work on all entities, collections, queries and region caches, by retrieving the list of names of entities, collections,queries and region caches using the following methods: getQueries(), getEntityNames(), getCollectionRoleNames(), and

getSecondLevelCacheRegionNames().

Chapter 21. Toolset Guide

Table of Contents


21.1.1. Customizing the schema21.1.2. Running the tool21.1.3. Properties21.1.4. Using Ant21.1.5. Incremental schema updates21.1.6. Using Ant for incremental schema updates21.1.7. Schema validation21.1.8. Using Ant for schema validation

Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and Ant tasks.

Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering of existingdatabases:

Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and syntax highlighting. Italso supports semantic auto-completion for class names and property/field names, making it more versatile than anormal XML editor.

Console: the console is a new view in Eclipse. In addition to a tree overview of your console configurations, you arealso provided with an interactive view of your persistent classes and their relationships. The console allows you toexecute HQL queries against your database and browse the result directly in Eclipse.

Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can use a wizard to quickly

generate Hibernate configuration (cfg.xml) files, or to reverse engineer an existing database schema into POJOsource files and Hibernate mapping files. The reverse engineering wizard supports customizable templates.

Please refer to the Hibernate Tools package documentation for more information.

However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka hbm2ddl.It can even be

used from "inside" Hibernate.


DDL can be generated from your mapping files by a Hibernate utility. The generated schema includes referential integrityconstraints, primary and foreign keys, for entity and collection tables. Tables and sequences are also created for mappedidentifier generators.

You must specify a SQL Dialect via the hibernate.dialect property when using this tool, as DDL is highly vendor-specific.

First, you must customize your mapping files to improve the generated schema. The next section covers schemacustomization.

21.1.1. Customizing the schema

Many Hibernate mapping elements define optional attributes named length, precision and scale. You can set the length,

precision and scale of a column with this attribute.

<property name="zip" length="5"/>

<property name="balance" precision="12" scale="2"/>

Some tags also accept a not-null attribute for generating a NOT NULL constraint on table columns, and a unique attribute for

generating UNIQUE constraint on table columns.

<many-to-one name="bar" column="barId" not-null="true"/>

<element column="serialNumber" type="long" not-null="true" unique="true"/>

A unique-key attribute can be used to group columns in a single, unique key constraint. The attribute overrides the name of

any generated unique key constraint.

<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/><property name="employeeId" unique-key="OrgEmployee"/>

A n index attribute specifies the name of an index that will be created using the mapped column or columns. Multiple

columns can be grouped into the same index by simply specifying the same index name.

<property name="lastName" index="CustName"/><property name="firstName" index="CustName"/>

A foreign-key attribute can be used to override the name of any generated foreign key constraint.

<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>

Many mapping elements also accept a child <column> element. This is particularly useful for mapping multi-column types:

<property name="name" type="my.customtypes.Name"/> <column name="last" not-null="true" index="bar_idx" length="30"/> <column name="first" not-null="true" index="bar_idx" length="20"/> <column name="initial"/></property>

The default attribute allows you to specify a default value for a column.You should assign the same value to the mapped

property before saving a new instance of the mapped class.

<property name="credits" type="integer" insert="false"> <column name="credits" default="10"/></property>

<version name="version" type="integer" insert="false"> <column name="version" default="0"/></property>

The sql-type attribute allows the user to override the default mapping of a Hibernate type to SQL datatype.

<property name="balance" type="float"> <column name="balance" sql-type="decimal(13,3)"/></property>

The check attribute allows you to specify a check constraint.

<property name="foo" type="integer"> <column name="foo" check="foo > 10"/></property>

<class name="Foo" table="foos" check="bar < 100.0"> ... <property name="bar" type="float"/></class>

The following table summarizes these optional attributes.

Table 21.1. Summary

Attribute Values Interpretation

length number column length

precision number column decimal precision

scale number column decimal scale

not-null true|false specifies that the column should be non-nullable

unique true|false specifies that the column should have a unique constraint

index index_name specifies the name of a (multi-column) index

unique-key unique_key_name specifies the name of a multi-column unique constraint

foreign-key foreign_key_name

specifies the name of the foreign key constraint generated for an association, for a<one-to-one>, <many-to-one>, <key>, o r <many-to-many> mapping element. Note that

inverse="true" sides will not be considered by SchemaExport.

sql-type SQL column type overrides the default column type (attribute of <column> element only)

default SQL expression specify a default value for the column

check SQL expression create an SQL check constraint on either column or table

The <comment> element allows you to specify comments for the generated schema.

<class name="Customer" table="CurCust"> <comment>Current customers only</comment> ...</class>

<property name="balance"> <column name="bal"> <comment>Balance in USD</comment> </column></property>

This results in a comment on table or comment on column statement in the generated DDL where supported.

21.1.2. Running the tool

The SchemaExport tool writes a DDL script to standard out and/or executes the DDL statements.

The following table displays the SchemaExport command line options

java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaExport options mapping_files

Table 21.2. SchemaExport Command Line Options

Option Description

--quiet do not output the script to stdout

--drop only drop the tables

--create only create the tables

--text do not export to the database

--output=my_schema.ddl output the ddl script to a file

--naming=eg.MyNamingStrategy select a NamingStrategy

--config=hibernate.cfg.xml read Hibernate configuration from an XML file

--properties=hibernate.properties read database properties from a file

--format format the generated SQL nicely in the script

--delimiter=; set an end of line delimiter for the script

You can even embed SchemaExport in your application:

Configuration cfg = ....;new SchemaExport(cfg).create(false, true);

21.1.3. Properties

Database properties can be specified:

as system properties with -D<property>

in hibernate.properties

in a named properties file with --properties

The needed properties are:

Table 21.3. SchemaExport Connection Properties

Property Name Description

hibernate.connection.driver_class jdbc driver class

hibernate.connection.url jdbc url

hibernate.connection.username database user

hibernate.connection.password user password

hibernate.dialect dialect

21.1.4. Using Ant

You can call SchemaExport from your Ant build script:

<target name="schemaexport"> <taskdef name="schemaexport" classname="org.hibernate.tool.hbm2ddl.SchemaExportTask" classpathref="class.path"/> <schemaexport properties="hibernate.properties" quiet="no" text="no" drop="no" delimiter=";" output="schema-export.sql"> <fileset dir="src"> <include name="**/*.hbm.xml"/> </fileset> </schemaexport></target>

21.1.5. Incremental schema updates

The SchemaUpdate tool will update an existing schema with "incremental" changes. The SchemaUpdate depends upon the

JDBC metadata API and, as such, will not work with all JDBC drivers.

java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaUpdate options mapping_files

Table 21.4. SchemaUpdate Command Line Options

Option Description

--quiet do not output the script to stdout

--text do not export the script to the database



--config=hibernate.cfg.xml specify a .cfg.xml file

You can embed SchemaUpdate in your application:

Configuration cfg = ....;new SchemaUpdate(cfg).execute(false);

21.1.6. Using Ant for incremental schema updates

You can call SchemaUpdate from the Ant script:

<target name="schemaupdate"> <taskdef name="schemaupdate" classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask" classpathref="class.path"/> <schemaupdate properties="hibernate.properties" quiet="no"> <fileset dir="src"> <include name="**/*.hbm.xml"/> </fileset> </schemaupdate></target>

21.1.7. Schema validation

Th e SchemaValidator tool will validate that the existing database schema "matches" your mapping documents. The

SchemaValidator depends heavily upon the JDBC metadata API and, as such, will not work with all JDBC drivers. This tool

is extremely useful for testing.

java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaValidator options mapping_files

The following table displays the SchemaValidator command line options:

Table 21.5. SchemaValidator Command Line Options

Option Description



--config=hibernate.cfg.xml specify a .cfg.xml file

You can embed SchemaValidator in your application:

Configuration cfg = ....;new SchemaValidator(cfg).validate();

21.1.8. Using Ant for schema validation

You can call SchemaValidator from the Ant script:

<target name="schemavalidate"> <taskdef name="schemavalidator" classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask" classpathref="class.path"/> <schemavalidator properties="hibernate.properties"> <fileset dir="src"> <include name="**/*.hbm.xml"/> </fileset> </schemavalidator></target>

Chapter 22. Additional modules

Table of Contents


22.1.1. Adding Bean Validation22.1.2. Configuration22.1.3. Catching violations22.1.4. Database schema


22.2.1. Description22.2.2. Integration with Hibernate Annotations

Hibernate Core also offers integration with some external modules/projects. This includes Hibernate Validator thereference implementation of Bean Validation (JSR 303) and Hibernate Search.


Bean Validation standardizes how to define and declare domain model level constraints. You can, for example, expressthat a property should never be null, that the account balance should be strictly positive, etc. These domain modelconstraints are declared in the bean itself by annotating its properties. Bean Validation can then read them and check forconstraint violations. The validation mechanism can be executed in different layers in your application without having toduplicate any of these rules (presentation layer, data access layer). Following the DRY principle, Bean Validation and itsreference implementation Hibernate Validator has been designed for that purpose.

The integration between Hibernate and Bean Validation works at two levels. First, it is able to check in-memory instancesof a class for constraint violations. Second, it can apply the constraints to the Hibernate metamodel and incorporate theminto the generated database schema.

Each constraint annotation is associated to a validator implementation responsible for checking the constraint on the entityinstance. A validator can also (optionally) apply the constraint to the Hibernate metamodel, allowing Hibernate to generateDDL that expresses the constraint. With the appropriate event listener, you can execute the checking operation on inserts,updates and deletes done by Hibernate.

When checking instances at runtime, Hibernate Validator returns information about constraint violations in a set ofConstraintViolations. Among other information, the ConstraintViolation contains an error description message that can embed

the parameter values bundle with the annotation (eg. size limit), and message strings that may be externalized to aResourceBundle.

22.1.1. Adding Bean Validation

To enable Hibernate's Bean Validation integration, simply add a Bean Validation provider (preferably HibernateValidation 4) on your classpath.

22.1.2. Configuration

By default, no configuration is necessary.

The Default group is validated on entity insert and update and the database model is updated accordingly based on the

Default group as well.

You can customize the Bean Validation integration by setting the validation mode. Use the javax.persistence.validation.mode

property and set it up for example in your persistence.xml file or your hibernate.cfg.xml file. Several options are possible:

auto (default): enable integration between Bean Validation and Hibernate (callback and ddl generation) only if Bean

Validation is present in the classpath.

none: disable all integration between Bean Validation and Hibernate

callback: only validate entities when they are either inserted, updated or deleted. An exception is raised if no Bean

Validation provider is present in the classpath.

ddl: only apply constraints to the database schema when generated by Hibernate. An exception is raised if no Bean

Validation provider is present in the classpath. This value is not defined by the Java Persistence spec and is specific toHibernate.

Note

You can use both callback and ddl together by setting the property to callback, dll

<persistence ...> <persistence-unit ...> ... <properties> <property name="javax.persistence.validation.mode" value="callback, ddl"/> </properties> </persistence-unit></persistence>

This is equivalent to auto except that if no Bean Validation provider is present, an

exception is raised.

If you want to validate different groups during insertion, update and deletion, use:

javax.persistence.validation.group.pre-persist: groups validated when an entity is about to be persisted (default to Default)

javax.persistence.validation.group.pre-update: groups validated when an entity is about to be updated (default to Default)

javax.persistence.validation.group.pre-remove: groups validated when an entity is about to be deleted (default to no group)

org.hibernate.validator.group.ddl: groups considered when applying constraints on the database schema (default to

Default)

Each property accepts the fully qualified class names of the groups validated separated by a comma (,)

Example 22.1. Using custom groups for validation

<persistence ...> <persistence-unit ...> ... <properties> <property name="javax.persistence.validation.group.pre-update" value="javax.validation.group.Default, com.acme.group.Strict"/> <property name="javax.persistence.validation.group.pre-remove" value="com.acme.group.OnDelete"/> <property name="org.hibernate.validator.group.ddl" value="com.acme.group.DDL"/> </properties> </persistence-unit></persistence>

Note

You can set these properties in hibernate.cfg.xml, hibernate.properties or

programmatically.

22.1.3. Catching violations

If an entity is found to be invalid, the list of constraint violations is propagated by the ConstraintViolationException which

exposes the set of ConstraintViolations.

This exception is wrapped in a RollbackException when the violation happens at commit time. Otherwise the

ConstraintViolationException is returned (for example when calling flush(). Note that generally, catchable violations are

validated at a higher level (for example in Seam / JSF 2 via the JSF - Bean Validation integration or in your business layerby explicitly calling Bean Validation).

An application code will rarely be looking for a ConstraintViolationException raised by Hibernate. This exception should be

treated as fatal and the persistence context should be discarded (EntityManager or Session).

22.1.4. Database schema

Hibernate uses Bean Validation constraints to generate an accurate database schema:

@NotNull leads to a not null column (unless it conflicts with components or table inheritance)

@Size.max leads to a varchar(max) definition for Strings

@Min, @Max lead to column checks (like value <= max)

@Digits leads to the definition of precision and scale (ever wondered which is which? It's easy now with @Digits :) )

These constraints can be declared directly on the entity properties or indirectly by using constraint composition.

For more information check the Hibernate Validator reference documentation athttp://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/


22.2.1. Description

Full text search engines like Apache Lucene are a very powerful technology to bring free text/efficient queries to

http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/

applications. If suffers several mismatches when dealing with a object domain model (keeping the index up to date,mismatch between the index structure and the domain model, querying mismatch...) Hibernate Search indexes yourdomain model thanks to a few annotations, takes care of the database / index synchronization and brings you backregular managed objects from free text queries. Hibernate Search is using Apache Lucene under the covers.

22.2.2. Integration with Hibernate Annotations

Hibernate Search integrates with Hibernate Core transparently provided that the Hibernate Search jar is present on theclasspath. If you do not wish to automatically register Hibernate Search event listeners, you can sethibernate.search.autoregister_listeners to false. Such a need is very uncommon and not recommended.

Check the Hibernate Search reference documentation ( http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/ ) for more information.

Chapter 23. Example: Parent/Child

Table of Contents

23.1. A note about collections23.2. Bidirectional one-to-many23.3. Cascading life cycle23.4. Cascades and unsaved-value

23.5. Conclusion

One of the first things that new users want to do with Hibernate is to model a parent/child type relationship. There are twodifferent approaches to this. The most convenient approach, especially for new users, is to model both Parent and Child as

entity classes with a <one-to-many> association from Parent to Child. The alternative approach is to declare the Child as a

<composite-element>. The default semantics of a one-to-many association in Hibernate are much less close to the usual

semantics of a parent/child relationship than those of a composite element mapping. We will explain how to use abidirectional one-to-many association with cascades to model a parent/child relationship efficiently and elegantly.

23.1. A note about collections

Hibernate collections are considered to be a logical part of their owning entity and not of the contained entities. Be awarethat this is a critical distinction that has the following consequences:

When you remove/add an object from/to a collection, the version number of the collection owner is incremented.

If an object that was removed from a collection is an instance of a value type (e.g. a composite element), that objectwill cease to be persistent and its state will be completely removed from the database. Likewise, adding a value typeinstance to the collection will cause its state to be immediately persistent.

Conversely, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not bedeleted by default. This behavior is completely consistent; a change to the internal state of another entity should notcause the associated entity to vanish. Likewise, adding an entity to a collection does not cause that entity to becomepersistent, by default.

Adding an entity to a collection, by default, merely creates a link between the two entities. Removing the entity will removethe link. This is appropriate for all sorts of cases. However, it is not appropriate in the case of a parent/child relationship. Inthis case, the life of the child is bound to the life cycle of the parent.

23.2. Bidirectional one-to-many

Suppose we start with a simple <one-to-many> association from Parent to Child.

<set name="children"> <key column="parent_id"/> <one-to-many class="Child"/></set>

If we were to execute the following code:

Parent p = .....;Child c = new Child();p.getChildren().add(c);session.save(c);session.flush();

Hibernate would issue two SQL statements:

an INSERT to create the record for c

an UPDATE to create the link from p to c

This is not only inefficient, but also violates any NOT NULL constraint on the parent_id column. You can fix the nullability

constraint violation by specifying not-null="true" in the collection mapping:

<set name="children"> <key column="parent_id" not-null="true"/> <one-to-many class="Child"/></set>

However, this is not the recommended solution.

The underlying cause of this behavior is that the link (the foreign key parent_id) from p to c is not considered part of the state

of the Child object and is therefore not created in the INSERT. The solution is to make the link part of the Child mapping.

http://lucene.apache.org

http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/

<many-to-one name="parent" column="parent_id" not-null="true"/>

You also need to add the parent property to the Child class.

Now that the Child entity is managing the state of the link, we tell the collection not to update the link. We use the inverse

attribute to do this:

<set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/></set>

The following code would be used to add a new Child:

Parent p = (Parent) session.load(Parent.class, pid);Child c = new Child();c.setParent(p);p.getChildren().add(c);session.save(c);session.flush();

Only one SQL INSERT would now be issued.

You could also create an addChild() method of Parent.

public void addChild(Child c) { c.setParent(this); children.add(c);}

The code to add a Child looks like this:

Parent p = (Parent) session.load(Parent.class, pid);Child c = new Child();p.addChild(c);session.save(c);session.flush();

23.3. Cascading life cycle

You can address the frustrations of the explicit call to save() by using cascades.

<set name="children" inverse="true" cascade="all"> <key column="parent_id"/> <one-to-many class="Child"/></set>

This simplifies the code above to:

Parent p = (Parent) session.load(Parent.class, pid);Child c = new Child();p.addChild(c);session.flush();

Similarly, we do not need to iterate over the children when saving or deleting a Parent. The following removes p and all its

children from the database.

Parent p = (Parent) session.load(Parent.class, pid);session.delete(p);session.flush();

However, the following code:

Parent p = (Parent) session.load(Parent.class, pid);Child c = (Child) p.getChildren().iterator().next();p.getChildren().remove(c);c.setParent(null);session.flush();

will not remove c from the database. In this case, it will only remove the link to p and cause a NOT NULL constraint

violation. You need to explicitly delete() the Child.

Parent p = (Parent) session.load(Parent.class, pid);Child c = (Child) p.getChildren().iterator().next();p.getChildren().remove(c);session.delete(c);session.flush();

In our case, a Child cannot exist without its parent. So if we remove a Child from the collection, we do want it to be deleted.

To do this, we must use cascade="all-delete-orphan".

<set name="children" inverse="true" cascade="all-delete-orphan"> <key column="parent_id"/> <one-to-many class="Child"/></set>

Even though the collection mapping specifies inverse="true", cascades are still processed by iterating the collection

elements. If you need an object be saved, deleted or updated by cascade, you must add it to the collection. It is notenough to simply call setParent().

23.4. Cascades and unsaved-value

Suppose we loaded up a Parent in one Session, made some changes in a UI action and wanted to persist these changes in

a new session by calling update(). The Parent will contain a collection of children and, since the cascading update is

enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in thedatabase. We will also assume that both Parent and Child have generated identifier properties of type Long. Hibernate will

use the identifier and version/timestamp property value to determine which of the children are new. (See Section 11.7,“Automatic state detection”.) In Hibernate, it is no longer necessary to specify an unsaved-value explicitly.

The following code will update parent and child and insert newChild:

//parent and child were both loaded in a previous sessionparent.addChild(child);Child newChild = new Child();parent.addChild(newChild);session.update(parent);session.flush();

This may be suitable for the case of a generated identifier, but what about assigned identifiers and composite identifiers?This is more difficult, since Hibernate cannot use the identifier property to distinguish between a newly instantiated object,with an identifier assigned by the user, and an object loaded in a previous session. In this case, Hibernate will either usethe timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if therow exists.

23.5. Conclusion

The sections we have just covered can be a bit confusing. However, in practice, it all works out nicely. Most Hibernateapplications use the parent/child pattern in many places.

We mentioned an alternative in the first paragraph. None of the above issues exist in the case of <composite-element>

mappings, which have exactly the semantics of a parent/child relationship. Unfortunately, there are two big limitations withcomposite element classes: composite elements cannot own collections and they should not be the child of any entityother than the unique parent.

Chapter 24. Example: Weblog Application

Table of Contents

24.1. Persistent Classes24.2. Hibernate Mappings24.3. Hibernate Code

24.1. Persistent Classes

The persistent classes here represent a weblog and an item posted in a weblog. They are to be modelled as a standardparent/child relationship, but we will use an ordered bag, instead of a set:

package eg;

import java.util.List;

public class Blog { private Long _id; private String _name; private List _items;

public Long getId() { return _id; } public List getItems() { return _items; } public String getName() { return _name; } public void setId(Long long1) { _id = long1; } public void setItems(List list) { _items = list; } public void setName(String string) { _name = string; }}

package eg;

import java.text.DateFormat;import java.util.Calendar;

public class BlogItem { private Long _id; private Calendar _datetime; private String _text; private String _title; private Blog _blog;

public Blog getBlog() { return _blog; } public Calendar getDatetime() { return _datetime; } public Long getId() { return _id; } public String getText() { return _text; } public String getTitle() { return _title; } public void setBlog(Blog blog) { _blog = blog; } public void setDatetime(Calendar calendar) { _datetime = calendar; } public void setId(Long long1) { _id = long1; } public void setText(String string) { _text = string; } public void setTitle(String string) { _title = string; }}

24.2. Hibernate Mappings

The XML mappings are now straightforward. For example:



<class name="Blog" table="BLOGS">

<id name="id" column="BLOG_ID">

<generator class="native"/>

</id>

<property name="name" column="NAME" not-null="true" unique="true"/>

<bag name="items" inverse="true" order-by="DATE_TIME" cascade="all">

<key column="BLOG_ID"/> <one-to-many class="BlogItem"/>

</bag>

</class>




<class name="BlogItem" table="BLOG_ITEMS" dynamic-update="true">

<id name="id" column="BLOG_ITEM_ID">

<generator class="native"/>

</id>

<property name="title" column="TITLE" not-null="true"/>

<property name="text" column="TEXT" not-null="true"/>

<property name="datetime" column="DATE_TIME" not-null="true"/>

<many-to-one name="blog" column="BLOG_ID" not-null="true"/>

</class>


24.3. Hibernate Code

The following class demonstrates some of the kinds of things we can do with these classes using Hibernate:

package eg;

import java.util.ArrayList;import java.util.Calendar;import java.util.Iterator;import java.util.List;

import org.hibernate.HibernateException;import org.hibernate.Query;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;import org.hibernate.tool.hbm2ddl.SchemaExport;

public class BlogMain { private SessionFactory _sessions; public void configure() throws HibernateException { _sessions = new Configuration() .addClass(Blog.class) .addClass(BlogItem.class) .buildSessionFactory(); } public void exportTables() throws HibernateException { Configuration cfg = new Configuration() .addClass(Blog.class) .addClass(BlogItem.class); new SchemaExport(cfg).create(true, true); } public Blog createBlog(String name) throws HibernateException { Blog blog = new Blog(); blog.setName(name); blog.setItems( new ArrayList() ); Session session = _sessions.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); session.persist(blog); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return blog; } public BlogItem createBlogItem(Blog blog, String title, String text) throws HibernateException { BlogItem item = new BlogItem(); item.setTitle(title); item.setText(text); item.setBlog(blog); item.setDatetime( Calendar.getInstance() ); blog.getItems().add(item); Session session = _sessions.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); session.update(blog); tx.commit();

tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return item; } public BlogItem createBlogItem(Long blogid, String title, String text) throws HibernateException { BlogItem item = new BlogItem(); item.setTitle(title); item.setText(text); item.setDatetime( Calendar.getInstance() ); Session session = _sessions.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); Blog blog = (Blog) session.load(Blog.class, blogid); item.setBlog(blog); blog.getItems().add(item); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return item; } public void updateBlogItem(BlogItem item, String text) throws HibernateException { item.setText(text); Session session = _sessions.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); session.update(item); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } } public void updateBlogItem(Long itemid, String text) throws HibernateException { Session session = _sessions.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); BlogItem item = (BlogItem) session.load(BlogItem.class, itemid); item.setText(text); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } } public List listAllBlogNamesAndItemCounts(int max) throws HibernateException { Session session = _sessions.openSession(); Transaction tx = null; List result = null; try { tx = session.beginTransaction(); Query q = session.createQuery( "select blog.id, blog.name, count(blogItem) " + "from Blog as blog " + "left outer join blog.items as blogItem " + "group by blog.name, blog.id " + "order by max(blogItem.datetime)" ); q.setMaxResults(max); result = q.list(); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return result; } public Blog getBlogAndAllItems(Long blogid) throws HibernateException { Session session = _sessions.openSession(); Transaction tx = null; Blog blog = null; try { tx = session.beginTransaction(); Query q = session.createQuery( "from Blog as blog " + "left outer join fetch blog.items " + "where blog.id = :blogid" ); q.setParameter("blogid", blogid); blog = (Blog) q.uniqueResult(); tx.commit(); } catch (HibernateException he) {

catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return blog; } public List listBlogsAndRecentItems() throws HibernateException { Session session = _sessions.openSession(); Transaction tx = null; List result = null; try { tx = session.beginTransaction(); Query q = session.createQuery( "from Blog as blog " + "inner join blog.items as blogItem " + "where blogItem.datetime > :minDate" );

Calendar cal = Calendar.getInstance(); cal.roll(Calendar.MONTH, false); q.setCalendar("minDate", cal); result = q.list(); tx.commit(); } catch (HibernateException he) { if (tx!=null) tx.rollback(); throw he; } finally { session.close(); } return result; }}

Chapter 25. Example: Various Mappings

Table of Contents

25.1. Employer/Employee25.2. Author/Work25.3. Customer/Order/Product25.4. Miscellaneous example mappings

25.4.1. "Typed" one-to-one association25.4.2. Composite key example25.4.3. Many-to-many with shared composite key attribute25.4.4. Content based discrimination25.4.5. Associations on alternate keys

This chapters explores some more complex association mappings.

25.1. Employer/Employee

The following model of the relationship between Employer and Employee uses an entity class (Employment) to represent the

association. You can do this when there might be more than one period of employment for the same two parties.Components are used to model monetary values and employee names.

Here is a possible mapping document:

<hibernate-mapping> <class name="Employer" table="employers"> <id name="id"> <generator class="sequence"> <param name="sequence">employer_id_seq</param> </generator> </id> <property name="name"/> </class>

<class name="Employment" table="employment_periods">

<id name="id"> <generator class="sequence"> <param name="sequence">employment_id_seq</param> </generator> </id> <property name="startDate" column="start_date"/> <property name="endDate" column="end_date"/>

<component name="hourlyRate" class="MonetaryAmount"> <property name="amount"> <column name="hourly_rate" sql-type="NUMERIC(12, 2)"/> </property> <property name="currency" length="12"/> </component>

<many-to-one name="employer" column="employer_id" not-null="true"/> <many-to-one name="employee" column="employee_id" not-null="true"/>

</class>

<class name="Employee" table="employees"> <id name="id"> <generator class="sequence"> <param name="sequence">employee_id_seq</param> </generator> </id> <property name="taxfileNumber"/> <component name="name" class="Name"> <property name="firstName"/> <property name="initial"/> <property name="lastName"/> </component> </class>


Here is the table schema generated by SchemaExport.

create table employers ( id BIGINT not null, name VARCHAR(255), primary key (id))

create table employment_periods ( id BIGINT not null, hourly_rate NUMERIC(12, 2), currency VARCHAR(12), employee_id BIGINT not null, employer_id BIGINT not null, end_date TIMESTAMP, start_date TIMESTAMP, primary key (id))

create table employees ( id BIGINT not null, firstName VARCHAR(255), initial CHAR(1), lastName VARCHAR(255), taxfileNumber VARCHAR(255), primary key (id))

alter table employment_periods add constraint employment_periodsFK0 foreign key (employer_id) references employersalter table employment_periods add constraint employment_periodsFK1 foreign key (employee_id) references employeescreate sequence employee_id_seqcreate sequence employment_id_seqcreate sequence employer_id_seq

25.2. Author/Work

Consider the following model of the relationships between Work, Author and Person. In the example, the relationship

between Work and Author is represented as a many-to-many association and the relationship between Author and Person is

represented as one-to-one association. Another possibility would be to have Author extend Person.

The following mapping document correctly represents these relationships:

<hibernate-mapping>

<class name="Work" table="works" discriminator-value="W">

<id name="id" column="id"> <generator class="native"/> </id> <discriminator column="type" type="character"/>

<property name="title"/> <set name="authors" table="author_work"> <key column name="work_id"/> <many-to-many class="Author" column name="author_id"/> </set>

<subclass name="Book" discriminator-value="B"> <property name="text"/> </subclass>

<subclass name="Song" discriminator-value="S"> <property name="tempo"/> <property name="genre"/> </subclass>

</class>

<class name="Author" table="authors">

<id name="id" column="id">  <generator class="assigned"/> </id>

<property name="alias"/> <one-to-one name="person" constrained="true"/>

<set name="works" table="author_work" inverse="true"> <key column="author_id"/> <many-to-many class="Work" column="work_id"/> </set>

</class>

<class name="Person" table="persons"> <id name="id" column="id"> <generator class="native"/> </id> <property name="name"/> </class>


There are four tables in this mapping: works, authors and persons hold work, author and person data respectively.

author_work is an association table linking authors to works. Here is the table schema, as generated by SchemaExport:

create table works ( id BIGINT not null generated by default as identity, tempo FLOAT, genre VARCHAR(255), text INTEGER, title VARCHAR(255), type CHAR(1) not null, primary key (id))

create table author_work ( author_id BIGINT not null, work_id BIGINT not null, primary key (work_id, author_id))

create table authors ( id BIGINT not null generated by default as identity, alias VARCHAR(255), primary key (id))

create table persons ( id BIGINT not null generated by default as identity, name VARCHAR(255), primary key (id))

alter table authors add constraint authorsFK0 foreign key (id) references personsalter table author_work add constraint author_workFK0 foreign key (author_id) references authorsalter table author_work add constraint author_workFK1 foreign key (work_id) references works

25.3. Customer/Order/Product

In this section we consider a model of the relationships between Customer, Order, Line Item and Product. There is a one-to-

many association between Customer and Order, but how can you represent Order / LineItem / Product? In the example,

LineItem is mapped as an association class representing the many-to-many association between Order and Product. In

Hibernate this is called a composite element.

The mapping document will look like this:

<hibernate-mapping>

<class name="Customer" table="customers"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <set name="orders" inverse="true"> <key column="customer_id"/> <one-to-many class="Order"/> </set> </class>

<class name="Order" table="orders"> <id name="id"> <generator class="native"/> </id> <property name="date"/> <many-to-one name="customer" column="customer_id"/> <list name="lineItems" table="line_items"> <key column="order_id"/> <list-index column="line_number"/> <composite-element class="LineItem"> <property name="quantity"/> <many-to-one name="product" column="product_id"/> </composite-element> </list> </class>

<class name="Product" table="products"> <id name="id"> <generator class="native"/> </id> <property name="serialNumber"/> </class>


customers, orders, line_items and products hold customer, order, order line item and product data respectively. line_items also

acts as an association table linking orders with products.

create table customers ( id BIGINT not null generated by default as identity, name VARCHAR(255), primary key (id))

create table orders ( id BIGINT not null generated by default as identity, customer_id BIGINT, date TIMESTAMP, primary key (id))

create table line_items ( line_number INTEGER not null, order_id BIGINT not null, product_id BIGINT, quantity INTEGER, primary key (order_id, line_number))

create table products ( id BIGINT not null generated by default as identity, serialNumber VARCHAR(255), primary key (id))

alter table orders add constraint ordersFK0 foreign key (customer_id) references customersalter table line_items add constraint line_itemsFK0 foreign key (product_id) references productsalter table line_items add constraint line_itemsFK1 foreign key (order_id) references orders

25.4. Miscellaneous example mappings

These examples are available from the Hibernate test suite. You will find many other useful example mappings there bysearching in the test folder of the Hibernate distribution.

25.4.1. "Typed" one-to-one association

<class name="Person"> <id name="name"/> <one-to-one name="address" cascade="all"> <formula>name</formula> <formula>'HOME'</formula> </one-to-one> <one-to-one name="mailingAddress" cascade="all"> <formula>name</formula> <formula>'MAILING'</formula> </one-to-one></class>

<class name="Address" batch-size="2" check="addressType in ('MAILING', 'HOME', 'BUSINESS')"> <composite-id> <key-many-to-one name="person" column="personName"/> <key-property name="type" column="addressType"/> </composite-id> <property name="street" type="text"/> <property name="state"/> <property name="zip"/></class>

25.4.2. Composite key example

<class name="Customer">

<id name="customerId" length="10"> <generator class="assigned"/> </id>

<property name="name" not-null="true" length="100"/> <property name="address" not-null="true" length="200"/>

<list name="orders" inverse="true" cascade="save-update"> <key column="customerId"/> <index column="orderNumber"/> <one-to-many class="Order"/> </list>

</class>

<class name="Order" table="CustomerOrder" lazy="true"> <synchronize table="LineItem"/> <synchronize table="Product"/> <composite-id name="id" class="Order$Id"> <key-property name="customerId" length="10"/> <key-property name="orderNumber"/> </composite-id> <property name="orderDate" type="calendar_date" not-null="true"/> <property name="total"> <formula> ( select sum(li.quantity*p.price) from LineItem li, Product p where li.productId = p.productId and li.customerId = customerId and li.orderNumber = orderNumber ) </formula> </property> <many-to-one name="customer" column="customerId" insert="false" update="false" not-null="true"/> <bag name="lineItems" fetch="join" inverse="true" cascade="save-update"> <key> <column name="customerId"/> <column name="orderNumber"/> </key> <one-to-many class="LineItem"/> </bag> </class> <class name="LineItem"> <composite-id name="id" class="LineItem$Id"> <key-property name="customerId" length="10"/> <key-property name="orderNumber"/> <key-property name="productId" length="10"/> </composite-id> <property name="quantity"/> <many-to-one name="order" insert="false" update="false" not-null="true"> <column name="customerId"/> <column name="orderNumber"/> </many-to-one> <many-to-one name="product" insert="false" update="false" not-null="true" column="productId"/> </class>

<class name="Product"> <synchronize table="LineItem"/>

<id name="productId" length="10"> <generator class="assigned"/> </id> <property name="description" not-null="true" length="200"/> <property name="price" length="3"/> <property name="numberAvailable"/> <property name="numberOrdered"> <formula> ( select sum(li.quantity) from LineItem li where li.productId = productId ) </formula> </property> </class>

25.4.3. Many-to-many with shared composite key attribute

<class name="User" table="`User`"> <composite-id> <key-property name="name"/> <key-property name="org"/> </composite-id> <set name="groups" table="UserGroup"> <key> <column name="userName"/> <column name="org"/> </key> <many-to-many class="Group"> <column name="groupName"/> <formula>org</formula> </many-to-many> </set></class> <class name="Group" table="`Group`"> <composite-id> <key-property name="name"/> <key-property name="org"/> </composite-id> <property name="description"/> <set name="users" table="UserGroup" inverse="true"> <key> <column name="groupName"/> <column name="org"/> </key> <many-to-many class="User"> <column name="userName"/> <formula>org</formula> </many-to-many> </set></class>

25.4.4. Content based discrimination

<class name="Person" discriminator-value="P"> <id name="id" column="person_id" unsaved-value="0"> <generator class="native"/> </id> <discriminator type="character"> <formula> case when title is not null then 'E' when salesperson is not null then 'C' else 'P' end </formula> </discriminator>

<property name="name" not-null="true" length="80"/> <property name="sex" not-null="true" update="false"/> <component name="address"> <property name="address"/> <property name="zip"/> <property name="country"/> </component> <subclass name="Employee" discriminator-value="E"> <property name="title" length="20"/> <property name="salary"/> <many-to-one name="manager"/> </subclass> <subclass name="Customer" discriminator-value="C"> <property name="comments"/> <many-to-one name="salesperson"/> </subclass> </class>

25.4.5. Associations on alternate keys

<class name="Person"> <id name="id"> <generator class="hilo"/> </id> <property name="name" length="100"/> <one-to-one name="address" property-ref="person" cascade="all" fetch="join"/> <set name="accounts" inverse="true"> <key column="userId" property-ref="userId"/> <one-to-many class="Account"/> </set> <property name="userId" length="8"/>

</class>

<class name="Address">

<id name="id"> <generator class="hilo"/> </id>

<property name="address" length="300"/> <property name="zip" length="5"/> <property name="country" length="25"/> <many-to-one name="person" unique="true" not-null="true"/>

</class>

<class name="Account"> <id name="accountId" length="32"> <generator class="uuid"/> </id> <many-to-one name="user" column="userId" property-ref="userId"/> <property name="type" not-null="true"/> </class>

Chapter 26. Best Practices

Write fine-grained classes and map them using <component>:

Use an Address class to encapsulate street, suburb, state, postcode. This encourages code reuse and simplifies

refactoring.

Declare identifier properties on persistent classes:

Hibernate makes identifier properties optional. There are a range of reasons why you should use them. Werecommend that identifiers be 'synthetic', that is, generated with no business meaning.

Identify natural keys:

Identify natural keys for all entities, and map them using <natural-id>. Implement equals() and hashCode() to compare the

properties that make up the natural key.

Place each class mapping in its own file:

Do not use a single monolithic mapping document. Map com.eg.Foo in the file com/eg/Foo.hbm.xml. This makes sense,

particularly in a team environment.

Load mappings as resources:

Deploy the mappings along with the classes they map.

Consider externalizing query strings:

This is recommended if your queries call non-ANSI-standard SQL functions. Externalizing the query strings tomapping files will make the application more portable.

Use bind variables.

As in JDBC, always replace non-constant values by "?". Do not use string manipulation to bind a non-constant valuein a query. You should also consider using named parameters in queries.

Do not manage your own JDBC connections:

Hibernate allows the application to manage JDBC connections, but his approach should be considered a last-resort.If you cannot use the built-in connection providers, consider providing your own implementation oforg.hibernate.connection.ConnectionProvider.

Consider using a custom type:

Suppose you have a Java type from a library that needs to be persisted but does not provide the accessors needed tomap it as a component. You should consider implementing org.hibernate.UserType. This approach frees the application

code from implementing transformations to/from a Hibernate type.

Use hand-coded JDBC in bottlenecks:

In performance-critical areas of the system, some kinds of operations might benefit from direct JDBC. Do not assume,however, that JDBC is necessarily faster. Please wait until you know something is a bottleneck. If you need to usedirect JDBC, you can open a Hibernate Session, wrap your JDBC operation as a org.hibernate.jdbc.Work object and

using that JDBC connection. This way you can still use the same transaction strategy and underlying connectionprovider.

Understand Session flushing:

Sometimes the Session synchronizes its persistent state with the database. Performance will be affected if thisprocess occurs too often. You can sometimes minimize unnecessary flushing by disabling automatic flushing, or evenby changing the order of queries and other operations within a particular transaction.

In a three tiered architecture, consider using detached objects:

When using a servlet/session bean architecture, you can pass persistent objects loaded in the session bean to andfrom the servlet/JSP layer. Use a new session to service each request. Use Session.merge() or Session.saveOrUpdate()

to synchronize objects with the database.

In a two tiered architecture, consider using long persistence contexts:

Database Transactions have to be as short as possible for best scalability. However, it is often necessary toimplement long running application transactions, a single unit-of-work from the point of view of a user. An applicationtransaction might span several client request/response cycles. It is common to use detached objects to implementapplication transactions. An appropriate alternative in a two tiered architecture, is to maintain a single openpersistence contact session for the whole life cycle of the application transaction. Then simply disconnect from theJDBC connection at the end of each request and reconnect at the beginning of the subsequent request. Never sharea single session across more than one application transaction or you will be working with stale data.

Do not treat exceptions as recoverable:

This is more of a necessary practice than a "best" practice. When an exception occurs, roll back the Transaction and

close the Session. If you do not do this, Hibernate cannot guarantee that in-memory state accurately represents the

persistent state. For example, do not use Session.load() to determine if an instance with the given identifier exists on

the database; use Session.get() or a query instead.

Prefer lazy fetching for associations:

Use eager fetching sparingly. Use proxies and lazy collections for most associations to classes that are not likely to becompletely held in the second-level cache. For associations to cached classes, where there is an a extremely highprobability of a cache hit, explicitly disable eager fetching using lazy="false". When join fetching is appropriate to a

particular use case, use a query with a left join fetch.

Use the open session in view pattern, or a disciplined assembly phase to avoid problems with unfetched data:

Hibernate frees the developer from writing tedious Data Transfer Objects (DTO). In a traditional EJB architecture,DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, theyimplicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOsbefore returning control to the presentation tier. Hibernate eliminates the first purpose. Unless you are prepared tohold the persistence context (the session) open across the view rendering process, you will still need an assemblyphase. Think of your business methods as having a strict contract with the presentation tier about what data isavailable in the detached objects. This is not a limitation of Hibernate. It is a fundamental requirement of safetransactional data access.

Consider abstracting your business logic from Hibernate:

Hide Hibernate data-access code behind an interface. Combine the DAO and Thread Local Session patterns. Youcan even have some classes persisted by handcoded JDBC associated to Hibernate via a UserType. This advice is,

however, intended for "sufficiently large" applications. It is not appropriate for an application with five tables.

Do not use exotic association mappings:

Practical test cases for real many-to-many associations are rare. Most of the time you need additional informationstored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate linkclass. In fact, most associations are one-to-many and many-to-one. For this reason, you should proceed cautiouslywhen using any other association style.

Prefer bidirectional associations:

Unidirectional associations are more difficult to query. In a large application, almost all associations must benavigable in both directions in queries.

Chapter 27. Database Portability Considerations

Table of Contents

27.1. Portability Basics27.2. Dialect27.3. Dialect resolution27.4. Identifier generation27.5. Database functions27.6. Type mappings

27.1. Portability Basics

One of the selling points of Hibernate (and really Object/Relational Mapping as a whole) is the notion of databaseportability. This could mean an internal IT user migrating from one database vendor to another, or it could mean aframework or deployable application consuming Hibernate to simultaneously target multiple database products by theirusers. Regardless of the exact scenario, the basic idea is that you want Hibernate to help you run against any number ofdatabases without changes to your code, and ideally without any changes to the mapping metadata.

27.2. Dialect

The first line of portability for Hibernate is the dialect, which is a specialization of the org.hibernate.dialect.Dialect contract. A

dialect encapsulates all the differences in how Hibernate must communicate with a particular database to accomplishsome task like getting a sequence value or structuring a SELECT query. Hibernate bundles a wide range of dialects formany of the most popular databases. If you find that your particular database is not among them, it is not terribly difficult towrite your own.

27.3. Dialect resolution

Originally, Hibernate would always require that users specify which dialect to use. In the case of users looking to

simultaneously target multiple databases with their build that was problematic. Generally this required their users toconfigure the Hibernate dialect or defining their own method of setting that value.

Starting with version 3.2, Hibernate introduced the notion of automatically detecting the dialect to use based on thejava.sql.DatabaseMetaData obtained from a java.sql.Connection to that database. This was much better, expect that this

resolution was limited to databases Hibernate know about ahead of time and was in no way configurable or overrideable.

Starting with version 3.3, Hibernate has a fare more powerful way to automatically determine which dialect to should beused by relying on a series of delegates which implement the org.hibernate.dialect.resolver.DialectResolver which defines only

a single method:

public Dialect resolveDialect(DatabaseMetaData metaData) throws JDBCConnectionException

The basic contract here is that if the resolver 'understands' the given database metadata then it returns the correspondingDialect; if not it returns null and the process continues to the next resolver. The signature also identifiesorg.hibernate.exception.JDBCConnectionException as possibly being thrown. A JDBCConnectionException here is interpreted

to imply a "non transient" (aka non-recoverable) connection problem and is used to indicate an immediate stop toresolution attempts. All other exceptions result in a warning and continuing on to the next resolver.

The cool part about these resolvers is that users can also register their own custom resolvers which will be processedahead of the built-in Hibernate ones. This might be useful in a number of different situations: it allows easy integration forauto-detection of dialects beyond those shipped with HIbernate itself; it allows you to specify to use a custom dialect whena particular database is recognized; etc. To register one or more resolvers, simply specify them (seperated by commas,tabs or spaces) using the 'hibernate.dialect_resolvers' configuration setting (see the DIALECT_RESOLVERS constant on

org.hibernate.cfg.Environment).

27.4. Identifier generation

When considering portability between databases, another important decision is selecting the identifier generation stratagyyou want to use. Originally Hibernate provided the native generator for this purpose, which was intended to selectbetween a sequence, identity, or table strategy depending on the capability of the underlying database. However, aninsidious implication of this approach comes about when targtetting some databases which support identity generationand some which do not. identity generation relies on the SQL definition of an IDENTITY (or auto-increment) column tomanage the identifier value; it is what is known as a post-insert generation strategy becauase the insert must actuallyhappen before we can know the identifier value. Because Hibernate relies on this identifier value to uniquely referenceentities within a persistence context it must then issue the insert immediately when the users requests the entitiy beassociated with the session (like via save() e.g.) regardless of current transactional semantics.

Note

Hibernate was changed slightly once the implication of this was better understood so thatthe insert is delayed in cases where that is feasible.

The underlying issue is that the actual semanctics of the application itself changes in these cases.

Starting with version 3.2.3, Hibernate comes with a set of enhanced identifier generators targetting portability in a muchdifferent way.

Note

There are specifically 2 bundled enhancedgenerators:

org.hibernate.id.enhanced.SequenceStyleGenerator

org.hibernate.id.enhanced.TableGenerator

The idea behind these generators is to port the actual semantics of the identifer value generation to the differentdatabases. For example, the org.hibernate.id.enhanced.SequenceStyleGenerator mimics the behavior of a sequence on

databases which do not support sequences by using a table.

27.5. Database functions

Warning

This is an area in Hibernate in need of improvement. In terms of portability concerns, thisfunction handling currently works pretty well from HQL; however, it is quite lacking in allother aspects.

SQL functions can be referenced in many ways by users. However, not all databases support the same set of functions.Hibernate, provides a means of mapping a logical function name to a delegate which knows how to render that particularfunction, perhaps even using a totally different physical function call.

Important

Technically this function registration is handled through theorg.hibernate.dialect.function.SQLFunctionRegistry class which is intended to allow users to

provide custom function definitions without having to provide a custom dialect. Thisspecific behavior is not fully completed as of yet.It is sort of implemented such that users can programatically register functions with theorg.hibernate.cfg.Configuration and those functions will be recognized for HQL.

http://in.relation.to/2082.lace

27.6. Type mappings

This section scheduled for completion at a later date...

References

[PoEAA] Patterns of Enterprise Application Architecture. 0-321-12742-0. Martin Fowler. Copyright © 2003 PearsonEducation, Inc.. Addison-Wesley Publishing Company.

[JPwH] Java Persistence with Hibernate. Second Edition of Hibernate in Action. 1-932394-88-5.http://www.manning.com/bauer2 . Christian Bauer and Gavin King. Copyright © 2007 Manning Publications Co.. ManningPublications Co..

http://www.manning.com/bauer2

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